ELECTRONIC MESSAGE FILTER HAVING A WHITELIST DATABASE AND A QUARANTINING MECHANISM

Abstract: An Active Filtering proxy (1104) Filters electronic junk maiL received at a Message Transfer Agent from remote Internet hosts using the Simple Mail Transfer Protocol (SMTP). The proxy (1104)actively probes remote hosts that attempt to send mail to the protected mail server in order to identify dialup PCs, open relays, and forged email. The system provides multiple layers of defense including: connect-time filtering based on IP address, identification of dialup PCs attempting to send mail, testing for permissive (open) relays, testing for validity of the sender's address and message header filtering. If a message passes through all relevant layers, it is delivered directly to all recipients. A recipient whitelist (1094) permits the user or system administrator to identify particular senders and/or domains as acceptable. If one or more recipients have agreed to receive mail from the sender, the message is delivered to those recipients and rejected or quarantined for the remainder of the recipients.

Description: ELECTRONIC MESSAGE FILTER HAVING A WHITELIST DATABASE AND A QUARANTINING MECHANISM This application is a continuation-in-part of U.S. Serial No. 09/548,322, filed on April 12, 2000, which is a continuation-in-part of U.S. Serial No. 09/447,590, filed November 23, 1999, which are incorporated herein by reference. BACKGROUND OF THE INVENTION Field of the Invention This invention generally concerns electronic messaging. In particular, the present invention concerns a system for filtering undesired electronic mail. Description of the Related Art Generally, the term "spam" has come to refer to posting electronic messages to news groups or mailing to addresses on an address list the same message an unacceptably large number (generally, 20-25) of times. As used herein, the term "spam" or "junk mail" refers to the sending of unsolicited electronic messages (or "email") to a large number of users on the Internet. This includes email advertisements, sometimes referred to as Unsolicited Commercial Email (UCE), as well as non-commercial bulk email that advocates some political or social position. A "spammer" is a person or organization that generates the junk mail. The principal objection to junk mail is that it is theft of an organization's resources, such as time spent by employees to open each message, classify it (legitimate vs. junk), and delete the message. Time is also spent by employees following up on advertising content while on the job. In addition, there is an increased security risk from visiting web sites advertised in email messages. Employees may also be deceived into acting improperly, such as to release confidential information, due to a forged message. Still yet, there is a loss of the network administrator's time to deal with spam and forged messages, as well as the use of network bandwidth, disk space, and system memory required to store the message. Finally, in the process of deleting junk mail, users may inadvertently discard or overlook other important messages. Another objection to junk mail is that it is frequently used to advertise objectionable, fraudulent, or dangerous content, such as pornography, illegal pyramid schemes or to propagate financial scams. Spam can also be a serious security problem. For instance, the recent Melissa virus and ExploreZip.worm have been spread almost exclusively via email attachments. Such viruses are usually dangerous only if the user opens the attachment that contains the malicious code, but many users open such attachments. Email may also be used to download or activate dangerous code, such as Java applets, Javascript, and ActiveX controls. Email programs that support Hypertext Markup Language (HTML) can download malicious Java applets or scripts that execute with the mail user's privileges and permissions. Email has also been used to activate certain powerful ActiveX controls that were distributed with certain operating systems and browsers. In this case, the code is already on the user's system, but is invoked in a way that is dangerous. For instance, this existing code can be invoked by an email message to install a computer virus, turn off security checking, or to read, modify, or delete any information on the user's disk drive. Both spammers, and those who produce malicious code, typically attempt to hide their identities when they distribute mail or code. Instead of mailing directly from an easily- traced account at a major Internet provider, they may, for instance, send their mail from a spam-friendly network, using forged headers, and relay the message through intermediate hosts. Consequently, the same mechanisms that can be used to block spam can also be used to provide a layer of protection for keeping malicious code out ofan organization's internal network. Simple Mail Transfer Protocol fSMTP Simple Mail Transfer Protocol (SMTP) is the predominant email protocol used on the Internet. As described in Request for Comments (RFC) 821, SMTP provides for the transfer of electronic mail from a sending SMTP agent to a receiving SMTP agent. SMTP is most commonly used with the Transmission Control Protocol/Internet Protocol (TCP/IP) to transfer email between Internet hosts known as Message Transfer Agents (MTAs). As shown in Figure 1, Internet mail operates at two distinct levels: the User Agent (UA) and the MTA. User Agent programs provide a human interface to the mail system and are concerned with sending, reading, editing, and saving email messages. Message Transfer Agents handle the details of sending email across the Internet. According to SMTP, an email message is typically sent in the following manner. A user 1040 (located at a personal computer or a terminal device) runs a UA program 1041 to create an email message. When the User Agent completes processing of the message, it places the message text and control information in a queue 1042 of outgoing messages. This queue is typically implemented as a collection of files accessible to the MTA. In some instances, the message may be created on a personal computer and transferred to the queue using methods such as the Post Office Protocol (POP) or Interactive Mail Access Protocol (IMAP). The sending network will have one or more hosts that run a MTA 1043, such as Unix sendmail by Sendmail, Inc. of California or Microsoft Exchange. By convention, it establishes a Transmission Control Protocol (TCP) connection to the reserved SMTP port (TCP 25) on the destination host and uses the Simple Mail Transfer Protocol (SMTP) 1044 to transfer the message across the Internet. The SMTP session between the sending and receiving MTAs results in the message being transferred from a queue 1042 on the sending host to a queue 1046 on the receiving host. When the message transfer is completed, the receiving MTA 1045 closes the TCP connection used by SMTP, the sending host 1043 removes the message from its mail queue, and the recipient 1048 can use his/her configured User Agent program 1047 to read the message in the mail queue 1046. Figure 2 is a graphical representation ofan example of the SMTP messages sent across the Internet. In this example, sender@remote.dom sends a message to user@escom.com (The top-level domain name "dom" does not actually exist, and is used for illustrative purposes only to avoid referring to a example domain). The sending host's Message Transfer Agent 1001 sends an email message to the receiving host 1002. At step 1010, the sending MTA opens a TCP connection to the receiving host's reserved SMTP port. This is shown as a dashed line with an italics description to differentiate it from the subsequent protocol messages. This typically involves making calls to the Domain Name System (DNS) to get the IP address of the destination host or the IP address from a Mail Exchange (MX) record for the domain. For example, the domain escom.com has a single MX record that lists the IP address 192.135.140.3. Other networks, particularly large Internet Service Providers (ISPs), might have multiple MX records that define a prioritized list of JP addresses to be used to send email to that domain. The sending MTA typically establishes the connection by: (1) making a socket system call to acquire a socket (a structure used to manage network communications); (2) filling in the socket structure with the destination IP address (e.g., 192.135.140.3); (3) defining the protocol family (Internet) and destination port number (by convention, the MTAs use the reserved TCP port 25); and, (4) making a connect system call to open a TCP connection to the remote MTA and returning a descriptor for the communications channel. The process of opening a TCP connection causes the receiving host's operating system (or networking software) to associate the TCP connection with a process that is listening on the destination TCP port. The TCP connection is a bi-directional pipe between the sending MTA 1001 on the sending host and the receiving MTA 1002 on the receiving host. SMTP is line-oriented, which means that all protocol messages, responses, and message data are transferred as a sequence of ASCII characters ending with a line feed (newline) character. In step 1011, the receiving MTA sends a service greeting message when it is ready to proceed. The greeting message typically gives the host name, MTA program and version number, date/time/timezone, and perhaps additional information as deemed by the host administrator. The greeting lines begin with the three-character numeric code "220". By convention, the last/only line begins with the four-character sequence "220 " and any preceding lines begin with "220-". When the greeting message is received, the sending MTA may optionally send a HELO message, step 1012, that lists its host name. Some mail servers require the sending host to issue this message, and others do not. If the client (sending) MTA issues the HELO message, then the server (receiving MTA) issues a HELO response, step 1013, that lists its name. For Extended SMTP (ESMTP), the sending host sends an EHLO message that performs essentially the same function as the HELO message. In this case, the receiving host generates a multi-line reply listing the extended SMTP commands that it supports. At step 1014, the sending MTA sends a MAIL From: message to identify the email address of the sender of the message, e.g., sender@remote.dom. By convention, the Internet address is formed by concatenating the sending user's account name, the "@" sign, and the domain name of the sending host. The resulting address is typically enclosed in angle-brackets, however, this is not usually required by the receiving mail server. It is noted that spammers can easily forge the MAIL address. At step 1015, the receiving mail server sends either a "250" response if it accepts the MAIL message or some other value such as "550", if the message is not accepted. The receiving mail server may reject the address for syntactical reasons (e.g., no "@" sign) or because of the identity of the sender. At step 1016, the sending MTA sends a RCPT To: message to identify the address ofan intended recipient of the message, e.g., user@escom.com. Again, this is a standard Internet address, enclosed in angle-brackets. At step 1017,. the receiving server replies with a "250" status message if it accepts the address, and some other value if the MAIL message is not accepted. For example, sendmail 8.9.3 issues a 550 message if the specified recipient address is not listed in the password file or alias list. The sending MTA may send multiple RCPT messages (step 1016), usually one for each recipient at the destination domain. The receiving server issues a separate "250" or "550" response as shown in step 1017 for each recipient. At step 1018, the sending mail server sends a DATA message when it has identified all of the recipients. The server sends a response (nominally, "354", as shown in step 1019) telling the sending server to begin sending the message one line at a time, followed by a single period when the message is complete. When the sending MTA receives this reply, it sends the text of the email message one line at a time as shown in step 1020. Note that it does not wait for a response after each line during this phase of the protocol. The message includes the SMTP message header, the body of the message, and any attachments (perhaps encoded) if supported by the sending User Agent program. When the message transfer has been completed, the sending MTA writes a single period (".") on a line by itself (step 1021) to inform the destination server of the end of the message. The receiving MTA typically responds (step 1022) with a "250" message if the message was received and saved to disk without errors. The sending MTA then sends a "quit" (step 1023) and the receiving MTA responds with a "221" message as shown in step 1024 and closes the connection. Figure 3 shows the same information, using a text representation of the SMTP messages between the sending MTA (remote.dom) and receiving MTA (escom.com). The first character of each line indicates the direction of the protocol message. The ">" character indicates the direction of the protocol message sent by the sending MTA, and "<" indicates the direction of a message sent by the receiving MTA. These characters do not form a part of the message being transmitted. The email message header is transferred at the beginning of the message and extends to the first blank line. As described in RFC 822, Standard for the Format of ARPA Internet Text Messages, the email message header includes Received: lines added by each MTA that received the message, the message timestamp, message ID, To and From addresses, and the Subject of the message. The message header is followed by the body of the message (in this case, a single line of text), the terminating period, and the final handshaking at the end of the message. Here, the term "message" alone refers to the overall email message as well as the multiple protocol messages (e.g., HELO, MAIL and RCPT) that are used by SMTP. Spammer Techniques The two primary techniques used by spammers are relaying and directing SMTP from a dialup PC. Approximately one-half of all spam attempts are relayed from an attacker through an intermediate site that permits relaying. Many of these open relay sites have been recently added to the Internet without regard to good system administration practices, and consequently may permit relaying without regard to its consequences. Approximately one-third of junk mail is sent directly from a dialup PC to the recipient mailhost. The use of direct SMTP from a PC provides the ability to forge email. As open relays are closed, this percentage is likely to rise. The remainder (approximately 15%) of junk mail is from users that appear to have an account on the sending network. Regardless of which technique is used, however, almost all junk mail have similar characteristics. Junk mail messages almost invariably have a forged email address in order to discourage complaints by the recipients of the spam. Contact information is provided somewhere in the body of the message, and may be another email address, a link to a web page or a telephone number. In addition, junk mail frequently does not include the recipient's address in the header of the message. This is done primarily as a performance optimization. In addition, junk mail is usually sent from a "throwaway" account, in which the spammer sends a batch of messages (usually thousands of messages) and then moves on after being canceled. Similarly, spamming networks sometimes perform spam runs from a mail server, then take the host offline to avoid complaints. Such networks operate until they are widely blacklisted, then register a new domain and carry on business under a different name. Any person with an email address at an Internet Service Provider (ISP) account can send junk email. After acquiring an address list, the user can send a message to each address on the list using the mailer program provided by the ISP. However, as shown in the examples in Figures 2 and 3, most ISPs record the sender's actual email address in outgoing message headers. If recipients complain, the ISP will often terminate the user's account, sometimes billing cleanup fees in accordance with the network's Acceptable Use Policy (AUP). Consequently, this technique is not favored by most spammers. Relay Abuse Relaying is not inherently bad. Early mailhosts relayed as a matter of courtesy and convenience for system administrators to test their mail systems. In addition, most networks relay internally so that not all network hosts have to be able to handle Internet mail. Small network subscribers often relay through a "smart host" provided by their ISP that is configured to handle the more complicated aspects of Internet mail. This arrangement is intentional and usually is not abused. The problem occurs when a host indiscriminately relays mail from any domain to any other domain. These hosts are known as "open relays". The practice is sometimes referred to as "third-party relaying", since the relay host is neither the initial sender of the message nor the intended recipient. Open relays permit the spammer to easily forge his/her identity. Figure 4 shows how a spammer at spam.dom 1060 relays mail via relay.dom 1061 to a variety of different users at different target domains 1062, 1074, etc. At step 1063, the spammer connects to relay.dom, as described with regard to Figure 2. For clarity, SMTP responses (greeting messages, 250, etc.) are not shown in this figure. At step 1064, the spammer forges a MAIL From message listing an address at the open relay host 1061. The forged MAIL address can be at any network, including spam.dom, relay.dom, any of the netn.dom hosts, or somewhere else. The forged MAIL From: address may be the same as the From: line in the message header, or it may be different. At one time spammers commonly forged addresses at AOL.COM or other large networks, because those networks were so well known, but legal action by AOL in particular has largely stopped that practice. The spammer is able to forge the MAIL address usually because he or she is able to override the normal user authentication functions, perhaps as a trusted user of a network server or as the operator of a single-user PC. At steps 1065, 1066, the spammer sends multiple RCPT messages with a list of destination addresses. Finally, step 1068, the spammer sends a DATA message, the text of the email message, a period, and a quit message to relay.dom. When relay.dom receives the message, it stores the message in its mail queues until it has forwarded the message to each of the target addresses, or until the message has timed out. If it cannot deliver a message, it will typically retry periodically (perhaps every 10 minutes or perhaps once per day). The relay host will usually keep undelivered messages in its queue for up to a week. The result is that spam.dom will send the message once and the relay host 1061 will forward a copy of the message to each host 1062, 1074 in the address list. For example, relay.dom will open a connection 1070 to host netl.dom 1062, send the MAIL message 1071, send the RCPT message 1072, and then send the text of the message 1073. The relay host 1061 repeats this process for host net2.dom 1074, as shown by steps 1075-1078, and any remaining target hosts (not shown). If spam.dom listed 100 different hosts in the RCPT addresses it sent to relay.dom, then relay.dom will attempt to send the message 100 times. The difficulty in filtering relayed junk mail is shown in part by this example. If the spammer 1060 forges the MAIL From address to match the relay host (e.g., "good@relay.dom") then as observed by netl.dom 1062, the message appears to be from a legitimate user at relay.dom. This example shows abuse of one open relay. The current generation of relaying tools will also permit the spammer to enter a list of open relay hosts, and the software will use different relays for different groups of addresses. Thus, different users at the same target network may receive spam relayed via different paths. The primary technique in blocking relayed spam involves databases of blacklisted IP addresses, which can be consulted by spam filtering software to determine whether the sending host is an open relay. For example, sendmail 8.9.3 provides an option to look up the IP address of the sending host in such a database, and reject the mail if the database indicates that the IP address is an open relay. Examples of online blacklist databases include, for instance, the Mail Abuse Prevention System (MAPS) Realtime Blackhole List (RBL) and the Internet Mail Relay Services Survey (IMRSS). The problem with such blacklisting databases is that they are static rather than dynamic. Consequently, an open relay must be abused at least once, reported to the database, confirmed by the database organization, then added to the database, before it will be blocked. Because database methods are static, the entry for a host must be manually removed when the host's mailer is fixed so it no longer relays. This takes an exchange of messages, re-testing, etc. In addition, these remote database methods involve connections to the database server, a lookup on that server (which may be doing lookups for hundreds of other users). Because these databases are global, they are not under control of local administrators. That is, if an organization has a customer that has an open relay, then the organization must either stop using blacklists such as MAPS or IMRSS, or risk having mail from the customer blocked because ofan entry in the MAPS or IMRSS databases. These database organizations typically take referrals from administrators throughout the Internet for open relay addresses. The organization then typically verifies the relay status before placing the address in the database. In the general case, an open relay can be confirmed by attempting to send a message from user A to user B, using the candidate relay address as an intermediate forwarder. The relay host may in turn relay the message through additional hosts in its network (known as "multi-hop" relaying), before sending it to user B. However, if user B eventually receives the message, then the host must have relayed. A much simpler test can be performed by simply telneting to the SMTP port (TCP 25) of the suspected open relay, then typing in SMTP commands as indicated in the ">" sequences in Figure 5 and observing the responses indicated in the "<" sequences. If the two networks are unrelated (i.e., the remote host is not acting as a legitimate smart host for the local organization) and the suspected relay host returns a "250" response to the RCPT message, then the remote host probably is an open relay. After the response to the RCPT message is received, the testing host can close the test connection without actually sending any data. However, this test is not perfectly accurate, as it fails to identify multi-hop relays. There are also some hosts that give "250" responses to the RCPT message, but actually reject the relay attempt during later mail processing. PC-based SMTP Direct Figure 6 shows how a spammer can use a dialup PC 1080 running a SMTP direct program 1081 that is able to establish SMTP connections 1044 directly to the SMTP port of the target mailhost. The term "dialup" as used herein refers to a class of Internet subscribers characterized by an inability to service incoming mail requests (i.e., not a mail server), having a related if not sequential name space, often using dynamically-assigned addresses, and generally existing at the lowest tier of pricing offered by an ISP. It includes various means of connecting, not all of which involve literally dialing in to the ISP, for example, wired cable or pocket radio. The spammer typically provides a single copy of a message 1082 and a list of addresses 1083. The program establishes an SMTP connection 1044 to each remote MTA 1045, delivers the message, and proceeds to the next entry in the address list. Because the Dialup SMTP Direct program 1081 runs under the control of the spammer, the program can be configured to forge any email address, hostname, or any field (e.g., the From: address) in the message header. Consequently, a message received by a user 1048 that is sent by this means may appear to be sent by a co-worker, from one's manager, from friends on another network, or even by the recipient himself. The primary method for blocking junk mail from SMTP Direct hosts is by using centralized blacklists. These include the MAPS Dialup User List (DUL). The DUL lists various blocks of IP addresses that are known to be used for dialup PCs. Current Solutions The solutions that are presently available to block junk mail fall into seven general categories. First, the use of centralized blacklisting databases, such as described above for the RBL, IMRSS, and DUL. Second, the use of local blacklisting databases, such as sendmail checking a local database and blocking email that matches entries in the database. Third, blocking mail from nonexistent domains, such as for instance if sendmail receives "MAIL From: <sender@nonexistent.dom>", it will reject the mail because it cannot find the domain "nonexistent.dom" listed in the Domain Name System (DNS). Fourth, whitelisting methods are used, so that a filter can reject all sender addresses that are not included in a local whitelist of permissible addresses. Fifth, Bcc filtering may be used to reject email from unknown hosts that do not list the recipient's email address in the header of the message. And sixth, client methods may be used to reject junk mail located in the user's mailbox without downloading the mail to the user's mail program (UA). Filtering of client protocols such as POP provides relief to individual users, but still allows junk mail to be stored on the SMTP server. Seventh, secure electronic mail, such as based on the emerging Secure/Multipurpose Internet Mail Extension (S/MIME) and OpenPGP standards uses public key cryptography to provide security services such as secrecy (confidentiality), integrity (ability to detect modification), authentication, and non-repudiation. Spammers are unlikely to use integrity and non-repudiation services, in particular, since these involve a digital signature signed with the sender's private key. However, these systems do not provide a solution to spam, since not everyone uses public key cryptography. Further, these services typically operate as part of the User Agent, so S/MXME or OpenPGP -protected spam can still be relayed or sent from dialup computers. SUMMARY OF THE INVENTION It is therefore a primary object of the invention to provide an email filtering system and method. It is another object of the invention to provide an email filtering system that substantially eliminates security risks and loss of company resources associated with junk mail. It is another object to provide an email filter that operates at the MTA level and performs active filtering based upon characteristics of the incoming connection and the remote host. In accordance with these objectives, an Active Filter proxy in accordance with a preferred embodiment is provided in a conventional firewall configuration between a remote host and a local MTA. The Active Filter proxy probes the sending host at the time it connects and implements a series of tests to determine if the remote host is likely to be either a dialup customer (Active Dialup testing), or an open relay (Active Relay testing). It also queries the mail server that handles email to the supposed sender of the message to determine if the mail server will accept email for that address (Active User testing). Together, these tests address the primary sources of junk mail. These tests reject SMTP email based on characteristics of the received SMTP protocol fields and the configuration of the remote host. The Active Dialup test considers certain characteristics typical of dialup PCs, which include the inability to operate as a server and generally a sequential naming scheme. The Active Relay test concludes that if the remote host appears to relay for a test connection, then it will probably relay for spammers. The Active User test detects obvious forgeries by blocking email where the configured mailhost for the sender will not accept a reply to that address. Because these techniques are performed at the time of the initial SMTP data connection, they characterize the remote host as it is configured at that time, thus avoiding the latency problems of static blacklisting databases. Further, rejected mail does not consume any disk storage on either the proxy host or the mailhost. Instead, the rejected message remains on the remote host, whether an open relay or dialup PC. Thus, junk mail that is blocked by at least one of these tests does not make it onto the local mail server or user clients. Consequently, it cannot be used to propagate viruses or other malicious code and it cannot distract the intended recipient from his/her work. The Active Filtering proxy can be chained with other content filtering proxies in a conventional fashion to reject other objectionable or malicious content in the body of the message. Minimal involvement is required by email administrators, when compared with the administrative cost of removing junk mail from mail servers, cleaning up after a virus or other malicious code attack, complaining about junk mail, and solving other problems. Administrator involvement generally consists of reviewing logs and adding IP address blocks and domain names to trusted databases where necessary. It is not practical, and perhaps not possible, to blacklist all current and future sources of spam, or to whitelist all benign sources of legitimate email, because the Internet grows and changes so quickly. However, it is readily possible for most administrators to define the relatively few (perhaps tens or hundreds) trusted domain names and to rely on the Active Filtering methods to characterize the remainder of the hosts that connect. The method also provides the ability to automatically append IP addresses detected by certain sensor points back into the IP filtering list, so that those hosts can be subsequently blocked by a simple IP lookup mechanism. This provides a performance improvement by quickly rejecting subsequent connections from IP addresses that have already been rejected by one of the Active Filtering tests. The present invention is compatible with all known SMTP MTAs. The architecture permits a natural separation of responsibilities for the proxy and the MTA. The proxy offloads the rejection of junk mail, so that the MTA need only consider legitimate email. The MTA may provide other conventional spam-filtering methods of its own (for example, rejecting non-existent MAIL From domains) or may reject mail because the RCPT user does not exist on the local network. These and other objects of the invention, as well as many of the intended advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts the general architecture for Internet electronic mail using the Simple Mail Transfer Protocol (SMTP). Figure 2 is a graphical representation ofan exchange of SMTP protocol messages involved in transferring a single electronic mail message from one MTA to another. Figure 3 is a printout of the message of Fig. 2, showing the protocol transactions, message header, and message body. Figure 4 shows how a bulk mail program takes advantage of an open relay host elsewhere on the Internet to store a single message and a list of addresses, causing the relay to forward the message to each address in the address list at recipient MTAs. Spammers typically use relaying to offload processing from their computer and obscure their involvement in sending the message. Figure 5 shows the SMTP messages used to perform a simple test of a remote host to determine if it is an open relay. Figure 6 shows how spammers may transfer mail directly from a SMTP direct program on a personal computer to the input port of a MTA. Spammers typically use this method to make message forgery easier and to avoid their network's controls on outgoing email. Figure 7 is a block diagram of the Active Filter proxy server system in accordance with the preferred embodiment. Figures 8-12 show specific architectures in accordance with the present invention when deployed with other email processing systems. Figure 8 shows the general architecture, in which the Active Filtering proxy is connected in a preferred embodiment as part of a firewall between the Internet 1100 and the organization's MTA. Figure 9 shows the proxy and MTA residing on the same computer. Figures 10 and 11 show the present invention implemented as part of a SMTP wrapper process or in the MTA itself, respectively. Figure 12 shows how a proxy may be chained with a content- filtering proxy for enhanced control over incoming email. Figure 13 shows an overview of the protocol transactions exchanged in transferring a single email message from a remote host, the Active Filtering proxy server, and the protected MTA. Figures 14-23 show the details of the protocol interactions and processing flow for the transfer of a single email message from a remote host 1400, through an Active Filtering proxy server 1401, to a local MTA 1402. Figure 14 shows the initial connection from the remote host to the proxy, a blacklist check, and display of a greeting message to the remote host. Figure 15 shows the processing of the remote host's HELO and MAIL transactions by attempting to open a reverse test connection 1418 to the remote host. Figure 16 shows the general framework for the Active Dialup test. Figure 17 shows details of the preferred embodiment for a sequential name check used in the Active Dialup test. Figure 18 shows the Active Relay test. Figure 19 illustrates the Active User verification method. Figure 20 shows how the proxy opens a connection to the local MTA 1403 to transfer a valid message. Figure 21 shows the transfer of the data in the email message (header, body, attachments, etc.) and how the connection is closed. Figure 22 illustrates an alternative embodiment for the Active Dialup test based upon edit distances between the remote host name and its neighbors' names. Figure 23 shows a second alternative embodiment for the Active Dialup test based upon the inability to establish reverse test connections to neighbors of the remote host. Figure 24 is a block diagram of the Active Filter proxy server system in accordance with the alternate preferred embodiment having an optional per-recipient whitelist database and quarantining. Figure 25 is an overview flow chart showing the processing of the MAIL From message with respect to the embodiment of Fig. 24. This includes the Active Filtering methods described in Figures 15-19, however, enforcement of the decision is made separately for each subsequent recipient identified in an RCPT message. Figure 26 is an overview flow chart of per-RCPT whitelist processing for an individual recipient. The proxy connects to the local MTA after the first authorized recipient is identified. Figure 27 shows how the proxy quarantines a message that did not pass Active Filtering and is not whitelisted for the current recipient. Figure 28 shows the processing of the remainder of the email message, beginning with the DATA transaction. An email message can be transferred directly to one group of recipients and also quarantined for the remainder of recipients. Figure 29 shows the retrieval of a quarantined message by a user or administrator, with the proxy transferring the quarantined message to the MTA as it would any other valid message. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. Architectures Figure 7 illustrates the design of the Active Filtering proxy server. The server runs as a process 1104 on a host computer (preferably a firewall host 1103 as shown in Figure 8), interposed between remote hosts on the Internet 1100 and a mailhost 1105. The proxy design requires services provided by the computer hardware platform 1091 and the operating system 1090. The hardware platform 1091 includes one or more processors, memory, disk storage, and network interfaces. The number of processors and amount of memory required depends upon the anticipated processing load. A small network might suffice with a single processor and 32 megabytes of random access memory (RAM), while a larger network might require multiprocessor implementation with hundreds of megabytes of RAM. The hardware platform provides disk storage for the program used to implement proxy 1104, operating system 1090, various configuration databases (1093 - 1098), and a log file 1099. Two network interfaces are preferred if the mailhost 1105 is to be hidden behind a firewall. The platform may also include a console (not shown) for configuring and controlling the server, however, this may also be performed via the network. The operating system 1090 provides an execution environment for the proxy process of proxy 1104 using the hardware 1091. It provides Transmission Control Protocol (TCP) socket services, Domain Name System (DNS) services, file system services, memory management services, and logging services. In modern operating systems (such as Solaris, Linux, AIX, and Windows NT), the file and memory management functions cooperate to provide access to a virtual memory space that exceeds the amount of physical memory available. Since the program image of proxy 1104 and all of the configuration files except the blacklist database 1095 and log 1099 are read-only, these may be read in once from disk then subsequently accessed from virtual memory. The operating system also provides the abstraction of TCP sockets 1092 and 1089. Each socket identifies a remote host endpoint, such that the socket 1092 is associated with a remote host (shown subsequently as Figure 13 item 1400) and the socket 1089 is used to control communications with the local Message Transfer Agent 1402 (Figure 13). An additional socket (not shown) is used for each test connection, e.g., 1418 (Figure 15) or 1903 (Fig. 19). The operating system also provides a means, such as the UNIX Internet Daemon (inetd), for dispatching programs (such as the proxy 1104) when connections are received from the Internet 1100. Configuration databases include Trusted DB 1093, which is used to identify trusted networks that are permitted to bypass further filtering; Whitelist DB 1094, which contains individual email addresses that are permitted to bypass further filtering; Blacklist DB 1095, which identifies IP addresses of remote hosts that will be blocked immediately after they connect to the proxy server; Relay DB 1096, which contains configuration data for the Active Dialup filter, including addresses of untrusted hosts that are known not to be dialup clients; Dialup DB 1097, which identifies untrusted hosts that are known not to be dialup clients; Configuration DB 1098, which includes general data such as the IP address and port for the Mailhost 1105, permissible domain names for RCPT messages, etc; and System Log 1099, as typically provided by the UNIX syslog facility or Windows NT Event Log service. The preferred embodiment is for each database to be provided as a separate file. However, alternative embodiments may provide for merging some or all databases into a single configuration database, however preferably excluding the Log 1099. Further to the preferred embodiment, the Active Filtering Proxy 1104 is run once for each incoming connection received from the Internet 1100, reads the configuration databases 1093-1098, interacts with the remote host to determine if it is likely to be a source of junk mail, and either closes the connection (without any mail being transferred) or opens a connection via the socket 1089 to permit the remote host to communicate with the mailhost 1105. In either case, the proxy writes one or more log entries to the Log 1099. The proxy does not save the message to a local file but instead performs all transfers from memory buffers. That is, the proxy receives a SMTP message from socket 1092 into a memory buffer, optionally validates the contents of the buffer, and then sends the contents of the buffer via socket 1089 to the mailhost 1105. In the preferred embodiment, the proxy exits after processing each message; however, alternative embodiments may provide for a single proxy that simultaneously handles multiple messages. Figures 8-12 show five architectures that provide Active Filtering of junk mail received from the Internet 1100 and addressed to an MTA 1106 on the organization's mail server 1105. The Active Filtering proxy 1104 (Fig. 7) runs on a separate firewall host 1103 (Fig. 8) or on the same mail server host 1107 that is running the MTA 1106 (Fig. 9). The methods can be implemented as part of a mail wrapper 1110 (Fig. 10), or can be integrated as part of the MTA 1113 itself (Fig. 11). If it runs as a separate proxy server 1103 (Fig. 8), it can be chained with other proxy servers 1116 to implement more complex filtering policies (Fig. 12). The organization's network includes, at a minimum, a router 1101, Internet connection 1100, Local Area Network (LAN) 1102 and MTA 1106. Accordingly, these components are common to all of the architectures shown in Figs. 8-12. The packet-filtering router 1101 routes packets from the Internet 1100 to the SMTP proxy server via the LAN 1102. The router operates at the network layer of the protocol reference model using the Internet Protocol version 4 (IPv4). However, with appropriate changes to the socket programming interface, the present invention also operates with other network layer protocols such as Internet Protocol version 6 (IPv6) or Novell Netware. Internet connection 1100, which is between the router 1101 and external hosts, is typically provided at the physical layer of the protocol hierarchy by wired or dialup circuits (such as dialup modem, ISDN, ADSL, or cable TV) using link-layer protocols such as Point- to-Point Protocol (PPP) or Single Line Interface Protocol (SLIP). The present invention operates at the application layer and is independent of the Internet connection. As with Figure 7, the firewall host 1103 has two separate LAN interfaces 1102 and 1117. LAN 1102 interconnects the Internet 1100 with the firewall host 1103. LAN 1117 connects the firewall host 1103 with the organization's protected servers (e.g., 1105) and workstations (not shown). These LANs are typically Ethernet or Token Ring technology. However, the present invention is independent of the type of LAN technology (adapters, device drivers) used by the organization. Each architecture of Figs. 8-12 has one or more MTAs 1106. These include programs such as Unix sendmail, Microsoft Exchange, Netscape Messaging Server, Lotus SMTP, Apple Internet Mail Server, special-purpose SMTP servers used by the various Internet service providers, and other MTA programs. These MTAs may themselves perform some degree of junk-mail filtering (for example, automated searches of the MAPS Realtime Blackhole List (RBL), MAPS Dialup User List (DUL), IMRSS relay list, or other databases. However, except as described for the Active Filtering MTA 1113, the methods described by the present invention are compatible with but do not depend on the choice of MTA 1106 or the MTA's spam-filtering mechanisms. Turning now to Figure 8, the Active Filtering mechanisms are implemented as an SMTP proxy server process 1104 on a dedicated firewall host 1103, as is typical in a firewall architecture. The proxy server is identified as a Mail Exchange (MX) host in the DNS information for the local organization and has bound port 25 so that all connections to port 25 of host 1103 will be directed to the proxy server process of filter 1104. Thus, when a remote host attempts to send mail to a user at the local network, the remote host gets the name of the proxy server from the MX record, translates the name into an IP address, acquires a socket, and opens a Transmission Control Protocol (TCP) connection to port 25 of the firewall host 1103, in accordance with standard socket programming practice. If the active probing of the remote host characterizes it as unlikely to be a source of spam, then the proxy server process 1104 opens a connection to port 25 of the mailhost 1105 (which has likewise been bound by the MTA 1106), transfers the initial protocol messages to the MTA 1106, and then transparently passes data to the MTA 1106. The router 1101, firewall host 1103 and mail server host 1105 can also be installed on a single LAN 1102. In this case, the firewall host 1103 has a single physical LAN interface device that is shared by the two logical interface functions (message arrival, via the router 1101, and message delivery to the mail server host 1105). The use of a shared physical LAN interface is conceptually the same as shown in Figure 8, with the exception that the firewall host 1103 cannot be configured to block packets from the Internet 1100 to the mail server host 1105. In this case, the router 1101 must be configured to block such direct access from the Internet to the mail server host 1105. With respect to Figure 9, the same Active Filtering proxy 1104 runs as a process on the mail server host 1107. The proxy 1104 performs the same functions as it does when it runs on a separate firewall host (i.e., Fig. 8), except that it does not necessarily need to establish an SMTP connection to the MTA 1106. Instead the proxy server 1104 may use any available InterProcess Communications (IPC) method 1108 that is provided by the mail server host 1107. Depending on the operating system, there are several alternatives for communicating with the MTA 1106. A first alternative, for instance, includes a TCP connection to some port other than TCP 25. Port 26 is used, although any other TCP port could be used. The router 1101 must then be configured to prevent packet communications directly to the selected port (e.g., 26). A second alternative with Unix hosts running sendmail, is for the proxy 1104 to be configured to save the message into a file, and pass the file to sendmail using the sendmail command line interface. Still yet, in a third alternative with Unix hosts, the proxy 1104 could use a Unix domain socket, named pipe, or other mechanism that is supported by the local MTA 1106. In Figure 10, the Active Filtering technology may be included as part of a MTA wrapper program 1110, for example, the Trusted Information Systems (TIS) Firewall Toolkit (FWTK) sendmail wrapper smap program. The smap program is essentially an SMTP proxy, but its primary function is not to block junk mail but rather to protect the sendmail program from attacks (such as unauthorized use of the DEBUG option), from stack overflow attacks, and from other external attacks on sendmail. Thus, the purpose of a wrapper 1110 is to protect the MTA 1106, and Active Filtering is an ancillary function. Various IPC methods 1108 are possible, although the FWTK smap program uses the sendmail command line interface. There is no need to do any special packet filtering, since the interface to sendmail is not visible to remote hosts. As shown in Figure 11, the Active Filtering technology 1112 could be implemented as part of a standard MTA 1106, resulting in a special MTA 1113 with Active Filtering. As shown in Figure 12, the Active Filtering proxy 1104 (on firewall host 1103) can be chained with other proxy servers 1116 (on firewall hosts 1114) to perform other mail filtering functions. For example, various products, such as in accordance with U.S. Patent No. 5,623,600, provide filtering of viruses and other malicious code. Preferably however, the Active Filtering proxy 1104 is the first host in the chain of proxies, that is, closest to the Internet, so it is best able to determine the essential characteristics of the remote host that is attempting to send email. The two filtering proxies 1104 and 1116 provide improved filtering by requiring each message to pass through both filters before it can be accessed at a client workstation. Provided the Active Filtering proxy has full access to the remote host, other configurations are possible. For instance, the Active Filtering and content filtering proxy servers (as well as the MTA) may run on the same proxy host. In addition, invocation of the content filtering proxy may use a means such as the Content Vectoring Protocol (CVP), rather than by serially linking the two proxies. This architecture permits additional proxies to be added to the chain, for example, proxies having other spam detection mechanisms or other content filtering techniques. With respect to Figures 8-12, there does not necessarily have to be a one-to-one relationship between the number of Active Filters and the number of MTAs within an organization. For example, in Figure 8 based upon performance and loading considerations, there might be three firewall hosts 1103, each connected to the LAN 1102, each running an Active Filtering Proxy process 1104, each having its own unique IP address, and each being configured as a MX host within the organization's DNS database. All three proxy servers 1103 would connect to the MTA 1106 only when they have legitimate (non-dialup, non-relayed, non- forged) email to deliver. While the individual active filtering processes themselves involve additional time and computing resources, these offload the processing of junk mail in such a way as to reduce the overall load on the MTA 1105. Within these various architectures, Active Filtering operates primarily as a server with respect to the initial connection from the remote host. SMTP is a client-server protocol, in which the remote host (client) issues requests to the local host (server). Although the remote host initiates the connection and each subsequent protocol exchange, transfer of the message is under control of Active Filtering proxy 1104, which may decide to reject a particular SMTP transaction or even disconnect from the remote host. The Active Filtering proxy 1104 (and its implementations in 1110 and 1112) provides for actively probing the remote host with a reverse SMTP connection to identify certain characteristics of the remote host that historically have a high correlation with sources of junk mail. Operation Figure 13 provides an overview of the present invention, with more detailed operation shown in Figs. 14-29. The figure shows the key steps used by the Active Filter Proxy 1401 to validate a single email message from a remote host 1400 and transfer the message to the protected MTA 1402. A separate SMTP connection 1418 is used for actively probing the remote host in order to perform Active Dialup 1420 detection and Active Relay 1450 detection. An additional connection may be established to a different mailhost for Active User testing. The Active Filter Proxy 1401 corresponds to proxy 1104 shown in Fig. 7. The proxy 1401 is shown in Fig. 13 connected between the remote host 1400 and the local MTA 1402. The proxy 1401 and MTA 1402 may be located at separate hosts, as shown in Figures 8 and 12, or at a same host as shown in Figures 9-11. Because the proxy 1401 controls when it reads data on the connection 1403, it is not possible for the remote host 1400 to proceed with transfer of its message until the proxy 1401 completes its filtering. The proxy only handles incoming email and does not process outgoing email from the MTA to remote hosts. Outgoing email is sent directly from the MTA 1402 to the network. With respect to Internet standards, the present invention may be implemented without any changes to SMTP or any other protocol. Rather, this method uses multiple SMTP connections, appropriately timed to permit the proxy server to characterize the remote host 1400. Thus, the SMTP connection 1403 is initiated by the remote host 1400, and involves transactions 1410, 1413, 1480, 1484, 1488, 1493, and 1495. The SMTP connection 1418 is initiated by the Active Filtering proxy 1401, and involves transactions beginning at step 1450. This session is used only to acquire protocol responses from the remote host 1400. It does not actually send an email message from the proxy server 1401 to the remote host 1400. In addition, the proxy server 1401 makes other connections to DNS name servers and, if the connection 1418 fails, may make an SMTP connection to the Mail Exchange (MX) host for the address given in step 1413. Taken together, the processing performed by the Active Filtering proxy 1401 involves the following actions when a remote host 1400 establishes a TCP connection 1403 to the proxy. First, as shown at step 1406, the proxy server 1401 gets the IP address of the remote host and compares the IP address with a database of disallowed addresses. If the IP address of the remote host 1400 matches an entry in the database, the proxy server closes the TCP connection 1403 without transferring an email message. This is described in greater detail in Figure 14. At steps 1410 and 1413, the proxy server processes the HELO (optional) and MAIL messages from the remote host 1400. The MAIL message contains the address of the purported sender of the incoming message, which is commonly forged in junk email. Except for trusted addresses (e.g. trusted hosts or whitelisted addresses) and other reverse test connections 1418 (to prevent cycles of reverse test connections), the proxy attempts to open a reverse test connection 1418 to the remote server host. The response (or lack of response) from the remote host dictates the subsequent processing flow. If the proxy cannot open the reverse connection, it may be because the remote host is a dialup workstation. Accordingly, the proxy then performs Active Dialup testing 1420. Internet service providers typically block service requests (such as SMTP) to their dialup customers using dynamic IP addresses (e.g., assigned by Dynamic Host Configuration Protocol, DHCP, which automatically assigns IP addresses to client stations logging onto a TCP/IP network). The proxy then uses certain heuristics based on the name of the host and its neighbors to categorize the host as a dialup or non-dialup. If it categorizes the host as a dialup, the proxy closes connections 1403 without transferring the email message. Otherwise, it performs Active User testing of the Mail Exchange (MX) host for the From address given in the MAIL message. Active Dialup is described more fully with respect to Figures 16-17. The administrator can configure the types of testing to be conducted by the proxy. The proxy reads the configuration database 1098 to determine the proper filtering modes. Thus, the administrator can set the configuration database 1098 to include flags for Active Dialup filtering, Active Relay filtering on a reverse connection, Active User filtering, Bcc filtering, and/or to append a filter to the blacklist database 1095 when any filter finds an email problem. The proxy filter will then conduct the appropriate filtering for the flags set in the configuration database 1098, but will not take any action for flags that are not set. If the reverse connection is successfully opened, then the proxy performs Active Relay 1450 testing. Under Active Relay testing 1450, once the reverse connection 1418 is opened, then the proxy 1401 sends HELO, MAIL From, and RCPT To messages to determine if the remote host would relay mail for the local proxy. If so, then it follows that the remote host is a high risk for relaying mail from other sources. If the reverse test messages 1450 indicate an open relay or the remote host rejects the MAIL From address 1413, the proxy preferably sends an error message and immediately closes the connection. Active Relay testing is discussed more fully with respect to Figure 18. If the results of the Active Dialup test are negative (that is, the proxy does not categorize the remote host as a dialup) or the results of the Active Relay test are indeterminate (the proxy is unable to successfully conclude Relay testing on that connection), then the proxy 1401 conducts Active User testing 1901. Here the proxy identifies a mailhost responsible for processing mail to the supposed sender of the message and queries that mailhost as to whether it will accept mail to that address. These protocol interactions are similar to those used in the Active Relay method but are not shown on Figure 13 since they do not usually involve the remote host 1400. If the configured mailhost for that address will not accept a reply to the MAIL From address, then the sender's address is probably forged, so the proxy 1401 sends an error message and immediately closes the connection. Active User testing is more fully discussed in relation to Figure 19. If the proxy 1401 does not reject the MAIL From transaction 1413 following the Active Dialup, Active Relay and/or Active User testing, then in step 1470 the proxy opens a data connection to the MTA 1402. In step 1472, if a HELO message was received in step 1410, then the proxy sends it to the MTA. In step 1474, the proxy sends the MAIL From message (received in step 1413) to the MTA, and sends the MTA response back to the remote host. This is more fully described with respect to Figure 20. Once the proxy 1401 opens the connection 1470 to the MTA 1402, it transfers protocol messages (e.g., RCPT 1480, DATA 1484, message data (header 1488 and body 1493), and the dot and quit messages 1495) to the MTA as they are received. This occurs transparently with the exception of a conventional Bcc filter 1491 that scans lines of the message header for To: or Cc: lines containing a local domain name. If it does not find such a header line, as is commonly done in junk mail messages, the Bcc filter 1491 returns an error to the remote host and closes all connections. The proxy 1401 also transfers MTA 1402 protocol responses (e.g., 250, 550, not shown) transparently to the remote host 1400. This is described in greater detail in Figures 20 and 21. When the message is transferred successfully, the MTA 1402 normally closes the connection to the proxy 1401, which in turn closes the connection to the remote host 1400. In single-threaded implementations, the proxy simply exits. In multi-threaded implementations, the proxy deallocates the resources (sockets, memory buffers, etc.) used for the message exchange and resets internal state variables to indicate that the message is no longer active. Figures 14-23 detail the methods and apparatus of the Active Filtering methods shown in Figure 13. These diagrams use a combination of the protocol message format favored in protocol documentation, and logic diagrams for the Active Filter proxy itself as is commonly used in software documentation. There are three primary participants throughout the protocol descriptions, namely Remote Host 1400, Active Filter Proxy 1401 and Local MTA 1402. Remote Host 1400, shown at the left in the figures, is the host that is attempting to send mail to the local domain. This host may be a sending MTA, a telnet session from a user shell account on the remote host, or a SMTP direct session from a user workstation. Active Filter Proxy 1401 is located between the Remote Host 1400 and Local MTA 1402, and is shown in the middle of the figures. Local MTA 1402 is shown at the right in the figures. The flow of mail from a legitimate host to the local protected MTA is shown in the figures as flowing from left to right. The system further interacts with DNS name servers, as well as the Sender's configured mailhost (Fig. 19). Referring back momentarily to Figures 8-12, the Active Filtering design may be implemented in various forms. Figures 13-21 apply to an Active Filter proxy server process that may or may not be located on the same host as the local MTA process 1402. The system is applicable to each embodiment of Figures 8-12 since the proxy 1401 and the MTA 1402 are assumed to interact via some form of Inter Process Communications (IPC) or intra-process communications for which the details of this IPC are irrelevant. In accordance with the preferred embodiment, the proxy runs on a separate host that is part of a firewall (Fig. 8). However, the proxy may run as two processes on the same host, using a TCP connection between the two processes. In all cases, the proxy host 1401 is preferably a Mail Exchange (MX) host for the local domain and is configured to listen on the SMTP port (TCP 25) for connections from remote hosts 1400. In the preferred embodiment, the proxy runs on a Unix system and the Unix inetd (Internet Daemon) program (not shown) is configured (via the /etc/inetd.conf file) to start a separate instance of the Active Filtering process when it receives the TCP connection to port 25. Thus, the proxy process 1401 handles a single message and exits when it has either rejected the message or transferred the message to the MTA. Connect-Time IP Address Filtering Operation of the filter proxy 1401 will now be described with reference to Fig. 14, which is after the proxy 1401 receives a connection 1403 from the remote host 1400. Starting at step 1404, the proxy 1401 gets the remote host's IP address and hostname from the Domain Name System (DNS). This is typically performed by calling the getpeername() function to get the 32-bit IP address of the connecting host and then converting it to a dotted-quad format (e.g., 192.168.200.201). The proxy then calls gethostbyaddr() to get the remote hostname (e.g., "smtp. remote.dom") from the IP address and calls gethostbyname() to verify the consistency of DNS information about the remote host. Properly configured hosts have a DNS Pointer (PTR) record that maps the IP address to a host name, and an Address (A) record that maps the name to the corresponding IP address. At the end of this step, the proxy has both the IP address and name (if defined in DNS), as well as an indicator as to the consistency of this information. At this stage, the proxy only acquires naming information about the remote host. It does not, at this point, decide to reject the message. In addition, the administrator can provide a filtering configuration rule that blocks mail from hosts that do not have a valid DNS configuration. In keeping with the general theme that most of the spam problem is because of misconfigured systems (misconfigured open relays, and the failure of ISPs to use their own packet routers to stop outgoing SMTP from their dialups), there are also many misconfigured name servers. So it is possible the proxy could get a connection from any servers in Table 1. Connecting host name from valid address from Host gethostbyaddrQ gethostbynameQ 192.168.200.200 abc.remote.dom 192.168.200.200 consistent info 192.168.200.201 smtp.remote.dom 192.168.200.99 inconsistent info 192.168.200.202 unavailable n/a incomplete info Table 1 At step 1405, the proxy determines if the remote host is categorized as trusted. Trusted networks are usually defined manually by using a suitable editor to enter IP addresses of trusted networks into the trusted database 1093 (Fig. 7). The proxy looks up the host name or IP address in a database of trusted network names and IP address blocks. This database is preferably a single linear file. A host name, e.g., "host37.remote.dom" matches an entry "remote.dom" if the two strings match from the last byte forward, for the length of the shorter string. If the host is trusted, processing continues with display of the greeting message in step 1409. If the host is not trusted, the system proceeds to step 1406, which is also shown in Fig. 13. Here, the proxy determines whether the remote network has been blacklisted. The proxy compares the IP address of the remote host 1400 with entries in the blacklist database. Preferably, the blacklist database is implemented as a linear file containing one filter per line. Each filter consists of an ASCII dotted-quad address followed by a forward slash "/" and the number of bits to be compared, for example, "192.168.200.201/24", with optional textual information such as the date the filter was created, the host name, and the reason. The proxy compares the remote host's IP address with a filter entry by converting the two IP addresses to 32-bit values, XORs the two values, and right shifts the result so that only the specified number of bits (e.g., 24) remain. If the result is zero, then the remote host 1400 matches that particular filter. However, the proxy can also provide other blacklisting approaches other than this type of long-term, IP-based blacklisting. For instance, the proxy can include blacklisting by domain name and short-term blacklisting for selected types of problems. Blacklisting by domain name is useful when an administrator observes a large amount of junk mail from a particular domain, e.g., ".KR" (Korea), but does not anticipate a need to receive any legitimate mail from those domains. In this case, the configuration database 1098 contains a list of patterns, and if the connection host name matches any of these patterns, the proxy closes the connection. Short-term blacklisting can be used to handle temporary situations (such as remote hosts with bad DNS configurations) as well as to limit bursts or retransmissions of junk mail when long-term blacklisting is not desirable. Short-term blacklisting uses an additional blacklist file that is periodically cleared out by the operating system. At step 1408, if the remote host is blacklisted, the proxy 1401 issues an error reply to the remote host (e.g., "550 SMTP administratively blocked"), closes the connection 1403, logs the rejected connection, and exits without any email being transferred. The system log 1099 (Fig. 7) may be configured to log on the local host or on a remote host, such as the local MTA 1402. If the remote host 1400 is trusted or the IP address acquired in 1404 does not match any entry in the blacklist 1406, then the Active Filter displays the SMTP greeting message, step 1409. Processing continues as shown in Figure 15 when the remote host sends data on the open connection 1403. At this point, the proxy has not established a connection to the local MTA 1402. The proxy connects to the server (Fig. 20) only after validating the MAIL From message. The use of linear files for the trusted database and the blacklist database might not be optimal for performance in all networks. Accordingly, trusted domain names (e.g., "remote.dom") might preferably be maintained in a hashed list or dbm file. Blacklisted IP addresses might preferably be maintained in bitmap, a hashed list, dbm file, or even in Content Addressable Memory (CAM) for increased performance. The check for a blacklisted IP address consists of opening the bitmap database, seeking to the appropriate byte, and reading the bit for the specified block (e.g., 192.135.140) of IP addresses. If the bit is set, then the block of addresses is blacklisted, otherwise it is acceptable. Blacklisted IP addresses are appended automatically to the blacklist database by various sensors in subsequent filters (i.e., the Active Dialup, Active Relay and Active User Filters), subject to a configuration setting. This permits the Active Filter proxy to react quickly to floods of spam from a particular host. However, if the sensor makes a bad decision, then the incorrect filter must be manually removed by editing the file. MAIL Message Processing At Figure 15, the remote host 1400 may send an optional HELO message 1410. In this event, the proxy 1401 simply reads the message in step 1411, potentially logs the message, and sends a response 1412 to the remote host. This message is irrelevant with respect to junk mail filtering, since no access decisions are made on the contents of the HELO message. At step 1413, the remote host 1400 sends a mandatory MAIL From message to the proxy 1401. At step 1414, the proxy reads the message from the TCP connection. The message must contain an email address, represented as "<mfaddr>", in the Internet address format consisting of the concatenation of a user name, "@" sign, and domain name. The term "MAIL From address" refers to the entire address passed in the MAIL From message, and the term "MAIL From domain" refers to the domain name to the right of the "@" sign. The filter proxy also ensures that the MAIL From addresses from selected large ISPs, such as AOL.com, HOTMAIL.com and YAHOO.com, must come from a host with the same name. This rejects a considerable amount of spam since spammers often forge addresses with well-known domain names. This aspect, however, is usually only useful for large ISPs. At step 1415, the proxy checks the MAIL From address to determine if the remote connection is from another Active Filtering proxy 1401. For instance, suppose host A and host B both have an Active Filtering proxy handling incoming email connections. Host A opens a data connection to host B. In turn, host B opens a reverse test connection back to host A. When this happens, host A must recognize the reverse test connection so that it does not propagate a cycle where each proxy opens reverse test connections to the other, until either the initial connection is terminated or one of the proxies runs out of resources. This can be handled in various ways, such as using a reserved MAIL From address, (e.g. "relaytest@hostb.somenet.dom") to explicitly indicate that this is a relay test. Alternatively, an Extended SMTP (ESMTP) command such as "XREVTEST" may be sent by host B to indicate that the connection is a reverse test connection. In accordance with the preferred embodiment of the invention, the Active Filtering proxy uses the reserved address "reverse" with the local domain name on each reverse test connection. This reserved address is used by all Active Filtering proxies. Continuing with step 1415, the proxy 1401 checks the MAIL From address to determine if it contains the reserved name "reverse" before the @ symbol. If so, the proxy issues an error reply 1416 on the incoming connection and exits. The receiving proxy then closes the connection when it detects this address to prevent abuse by spammers who might learn this reserved address. In this case, the remote host (e.g., the proxy at host B) will not be able to test the local host (e.g., host A), but email will still be possible. At step 1417, the proxy filter 1401 skips subsequent checking of the MAIL From argument if the connecting hostname matches a trusted database entry, using the same method as in step 1405, or if the MAIL From address matches an entry in the system whitelist. The trusted database identifies networks with which there are long-term trust relationships, so that any user from one of these domains can send mail without restriction. If the domain is trusted, step 1417, processing continues with step 1470. Also at step 1417, the filter skips subsequent checking of the MAIL From argument if the MAIL From address (user@domain) exactly matches an entry in the whitelist database. In the preferred embodiment, the whitelist file is a text file that contains addresses (one per line) that are periodically mined from sendmail log entries for outgoing ("to=") messages. These log entries are for mail sent by the local organization to destination addresses on other networks, so adding these destination addresses to the whitelist file will ensure that the proxy will permit incoming email from those persons that local users have sent mail to. However, the whitelist database maybe implemented as a hashed database (e.g., dbm) files, or even could be disabled. If the address matches a whitelist entry, processing continues with step 1470. The difference between the trusted database 1093 and the whitelist database 1094 is that for trusted hosts, mail is permitted from any user on the remote host to any user on the local host. For whitelist entries, mail is permitted only from the named user on the remote host to any user on the local host. If the incoming connection is not itself a relay test and the message does not match any of the trust criteria, then in step 1418, the proxy 1401 attempts to open the reverse test connection to the remote host 1400. This is typically performed by calling socket() to acquire a socket structure to manage the connection to port 25 of the remote IP address 1400, then calling connect() to request the networking software to establish a TCP connection using the socket. In step 1419, the proxy 1401 checks the status of the connect() call. TCP networking implementations by convention return a status of zero if the connection is successful, otherwise TCP returns -1 and sets an error number to indicate the specific error. If the reverse connection is successful, then the proxy continues with step 1450 to perform Active Relay testing (Fig. 18). However, if the reverse connection fails, then the proxy continues with Active Dialup testing in step 1420 (Fig. 16). In this case, the connection must have been blocked by the remote network (the remote host 1400 cannot be offline since it has just connected to the proxy server). In most of these cases the connection will be actively refused by the remote host 1400 (or its packet routers). This will result in an error of ECONNREFUSED 61. In a few cases, the remote network may silently block the TCP open request to the remote host 1400, without giving an error response. In this case, the local networking software will return ETIMEDOUT 60 as the network error. Active Dialup Categorization As noted above, email from dialup PCs running direct SMTP programs is a major problem since the spammer can use the program to forge any protocol field or message header field. Approximately one-third of the junk mail attempts are from ISP dialup addresses. The spammer almost always uses a relatively inexpensive "throwaway" dialup account with an Internet service provider (ISP). These dialup accounts typically have certain characteristics imposed by their respective ISPs. Because of the use of dynamic name allocation (e.g., DHCP) and because of pricing strategies, the ISP permits the user to only operate as a client. That is, the ISP uses its packet routers to block network service requests such as SMTP to their dialup users. The second characteristic of dialup accounts is that most ISPs use a regular naming scheme for such dialup addresses so as to simplify maintenance of the DNS database. The names frequently include decimal or hexadecimal representations of the last byte of the IP address. In Figure 16, the proxy 1401 continues with Active Dialup detection at step 1420 after determining at step 1419 that it cannot establish a reverse test connection to the remote host 1400. This method is performed only for untrusted hosts, since the reverse connection is not attempted for trusted hosts, step 1417. At step 1421, the proxy 1401 attempts to determine if the IP address or domain name matches a non-dialup entry in the dialup database. The Dialup configuration database 1097 (Figure 7) lists blocks of non-dialup addresses that otherwise meet the criteria for dialup (i.e., will not accept a reverse connection and have a sequential naming scheme) but that are known to not be dialups. For example, an ISP may have sequentially-named mailhosts with some mailhosts dedicated for outgoing mail and some mailhosts dedicated for incoming mail. To continue with step 1421, there will typically be only a few entries in this database because most non-dialups are characterized correctly by the Active Dialup method. The remaining entries are common across the Internet and can be pre-defined and installed along with the proxy server. It may be necessary to add an entry to this database whenever any ISP installs or renames a block of mailhosts that appear much like dialups, but this too can be centrally distributed. It is preferable to list these few address blocks than to attempt to identify all possible dialup addresses. The addresses in step 1421 are preferably expressed as a dotted-quad IP address, a slash "/", and a number of bits to be matched. For example, the filter 192.168.200.201/24 matches all addresses between 192.168.200.0 and 192.168.200.255. An address matches a particular filter if the filter address 1097 (Fig. 7) and the remote host 1400 IP address match for the specified number of bits. For example, the IP address 192.168.200.29 matches the filter 192.168.200.201/24 because the two addresses are identical for the first 24 bits, i.e., 192.168.200. The preferred embodiment uses a flat ASCII file structure for the dialup database. If the requirement for non-dialup entries grow significantly, other representations (hashed lists, dbm files, or CAM) may be desirable for performance reasons. If the IP address matches any entry, then the proxy 1401 bypasses any further dialup testing, and proceeds to step 1901. Relay testing is not conducted since the filter has already determined that the reverse connection cannot be established to the remote host at step 1419. If it does not match any entry in the non-dialup list, then it proceeds with dialup testing in step 1422. At steps 1422-1424, the proxy 1401 compares the name of the connecting host with its immediate neighbors, using a heuristic approach to correlate a sequence of names as dialups or non-dialups. In the preferred embodiment, a threshold total often match points are required to classify a remote host as a dialup. This approach takes into account the remote host name, character sequences in the name, and sequential nature of host names near the IP address of the remote host 1400. At step 1422, the filter scans the node name of the remote host 1400 for certain sequences and adds or subtracts points. The node name is the part of the host name up to the first period. For example, the node name for "dial-37.remote.dom" is "dial-37". The preferred embodiment obtains this information from an entry in the dialup configuration database 1097 (Fig. 7), which contains text strings and associated points, separated by a slash "/", as listed in Table 2. dial/5 PPp/5 slip/5 dhcp/5 smtp/-5 mail -5 TABLE 2 In the preferred embodiment of Table 2, five points are assigned if the node name contains "dial", "ppp" (for Point-to-Point protocol), "slip" (for Single Line IP), or "dhcp" (for Dynamic Host Configuration Protocol). Five points are subtracted if the node name contains "smtp" (indicating a SMTP host) or "mail" (indicating a mailhost). Of course, other sequences and point values may also be used. The above configuration data can be extended to include other sequences that may subsequently be associated with dialup hosts. Not all remote hosts have a consistent name and IP address assigned in DNS, as described for step 1404. That is, a call to gethostbyaddr() or gethostbyname() will fail, or the returned information is not consistent. The Dialup DB 1097 has a Reject- unknown-dial option to either reject the connection with an error message indicating a DNS naming error or continue processing, thus relying on other filter layers to catch the problem. At step 1423, the proxy 1401 compares the node name of the remote host 1400 with its neighbors and assigns additional points if the names appear to follow a sequential naming scheme. Further to the preferred embodiment, the proxy compares names of neighbor hosts by performing the following actions for all IP addresses that are within the range nnn-10 to nnn+10, where nnn is the node address (last byte of IP address) of the remote host 1400. Details of step 1423 are provided in Fig. 17. For example, the following Table 3 for a remote host ICustl 17.tnt5.salt-lake-city.ut.da.uu.net at IP address 63.11.217.117, shows the IP addresses and node names for its 20 nearest neighbors. Offset IP Address Node Name -10 63.11.217.107 lcustl07 -9 63.11.217.108 lcustl08 -8 63.11.217.109 lcustl09 -7 63.11.217.110 lcustl lO -6 63.11.217.111 lcustl l l -5 63.11.217.112 lcustl l2 -4 63.11.217.113 lcustl l3 -3 63.11.217.114 lcustl l4 -2 63.11.217.115 lcustl l5 -1 63.11.217.116 lcustl lό 0 63.11.217.117 lcustl l7 1 63.11.217.118 lcustl lδ 2 63.11.217.119 lcustl l9 3 63.11.217.120 lcustl20 4 63.11.217.121 lcustl21 5 63.11.217.122 lcustl22 6 63.11.217.123 lcustl23 7 63.11.217.124 lcustl24 8 63.11.217.125 lcustl25 9 63.11.217.126 lcustl26 10 63.11.217.127 lcustl27 Table 3 - Neighboring IP Addresses for Remote Host 63.11.217.117 This example shows how this particular ISP sequentially named its hosts over the range to be considered (and, indeed, throughout the entire block of addresses). In this case the last byte of the IP address is identified directly in the node name, but this is not necessary for this approach to work. The proxy can consider either node names or complete host names in evaluating whether the remote host exists within a sequential name space. In general, it is more efficient to consider node names, however, an ISP can organize a dialup name space so that the sequential naming scheme occurs within an intermediate node of the name, such as the IP addresses 24.65.51.66 and 24.65.51.67 for the names 24.65.51.66.on.wave.home.com and 24.65.51.67.on.wave.home.com, respectively. At step 1424, the proxy 1401 compares the total current number of match points from steps 1422 and 1423 with the threshold number of points (10, in the preferred embodiment) required to characterize the remote host as a dialup. If the number of match points exceeds the threshold, then it exits, step 1425. Otherwise, message transfer continues with step 1901. At step 1425, the proxy 1401 issues an SMTP error message (e.g., "550 apparent dialup") and exits, thus closing the data connection without any email being transferred. In the preferred embodiment, the proxy also logs the rejected dialup and adds the IP address of the remote host to the blacklist database. Figure 17 shows further detail of the processing flow for step 1423 in accordance with the preferred embodiment. Step 1500 calculates a 32-bit IP address for the remote host, which is used in step 1504 to calculate the IP address of one of its 20 neighbors. Steps 1501, 1502, and 1503 perform the remaining steps shown in the figure for x=-10 to x=+10, inclusive, while skipping the remote host at x=0. When the loop is finished, the proxy exits to step 1424 of Fig. 16, which classifies the remote host as a dialup or non-dialup based on the accumulated number of match points. Step 1505 limits the name comparison to the 8-bit (Class C) address block that contains the remote host to avoid comparing a remote host name in one ISP with neighbors in a block operated by a different ISP. It XORs the 32-bit IP address for the neighbor x and the IP address for the remote host and shifts the result right 8 bits. If the result is non-zero, then the neighbor x is in a different address block than the remote host, and is skipped. Thus, the range is absolutely bounded by a minimum node address of 0 and a maximum node address of 255, so that the comparison for remote host 192.168.200.2 would only consider node addresses from 0-1 and 3-12, in order to avoid comparing names in other 8-bit blocks of addresses. At a minimum, ten addresses will always be considered. Preferably, 10 names are matched out of 20 (n-10 to n+10) so that if the remote host is at the beginning of a block, e.g., 192.135.140.0, then there will still be ten opportunities to match from 1 to 10, and if the remote host is at the end of a block (e.g., 192.135.140.255, there will still be ten opportunities to match in the range 245 to 254. Steps 1506 and 1507 call gethostbyaddr() to get the host structure for the neighbor x, which contains the host name. Errors do not terminate the comparison, since there may be gaps in the DNS information near the remote host. Steps 1509 and 1510 compare the respective lengths of the remote host name and its neighbor x. If either is more than one character longer than the other, then skip the neighbor x because the two names do not appear to be part of a sequence. Step 1511 scans forward and backwards to identify the sequence of non-matching characters in the names of the remote host and its neighbor x. This sequence may contain substrings of matching characters, but as shown in step 1512, if either string is greater than three characters in length, then skip the neighbor x because the two names do not appear to be part of a sequence. Step 1513 scans the two strings from the names of the remote host and the neighbor x to determine if either contains a hexadecimal-only digit, i.e., a character in the range a-f. If so, it sets the hexmode flag. In step 1514-1516, the proxy 1401 checks the hexmode flag and converts the string for the host x to a hexadecimal or decimal value, based upon the setting of the hexmode flag. In step 1517-1519, the proxy calculates the absolute distance between the two name sequences. If the distance is less than or equal to the absolute value of x, then the names appear to be part of a sequence and the match counter is incremented. For example, Table 4 shows the distance as correlated to the offset x for the four nearest neighbors of the remote host 63.11.217.117, based on the information in Table 3. As shown in this example, the distance calculated for each of the four nearest neighbors is identically equal to the difference in IP address values, thus the names are part of a sequence. IP Offset (x) IP Address Node Name Distance -2 63.11.217.115 lcustl l5 2 -1 63.11.217.116 lcustl lό 1 0 63.11.217.117 lcustl l7 skipped 1 63.11.217.118 lcustl lδ 1 2 63.11.217.119 lcustl l9 2 Table 4 - Distance for IP Addresses Neighboring Remote Host 63.11.217.117 The preferred embodiment detailed in Figure 17 provides categorization of a dialup host based upon a linear correlation of its neighbors' host names. It does not require each host name to directly encode its IP address (as shown above), but also permits other linear relationships. It correctly handles address blocks that have one or more legitimate mailhosts, usually at the beginning of the block, but use the rest of the block for dialup addresses. It permits discontinuities in the name space, provided the remote host is part of a name sequence that is sufficiently long. It correctly handles node addresses at the top (e.g., 255) and bottom (e.g., 0) of a Class C block of addresses, where name discontinuities are most likely to occur. It also permits fixed-width names (e.g., "001" through "255") and variable-width names (e.g., "1" through "255"). And, as indicated in Figure 16, it permits either decimal or hexadecimal encodings. The preferred embodiment supplements these Active Dialup methods with the blacklist filter 1406 (Figure 14). Bulk mailers who use the SMTP direct mechanism will typically retry from different (dynamically assigned) IP addresses, but frequently from addresses in the same Class C (8-bit) address range. By adding the IP address to the blacklist database, nominally with the number of bits to be matched set to 24, the mechanism takes advantage of the relative speed of a blacklist database lookup as compared with subsequent iterations of the above active dialup mechanism. The filter can be left perpetually in the blacklist database, or preferably removed from the blacklist database if it is not used in some number of days or weeks. Further, filters can be manually added to handle any blocks of dialup addresses that are not identified by the Active Dialup method. The preferred embodiment provides for Active Dialup detection following reception of the MAIL From message on connection 1403. This permits logging of the MAIL From address and also prevents continuous cycles of reverse test connections since Active Dialup detection occurs after the reverse connection test in step 1415. However, in an alternate embodiment a proxy might perform Active Dialup detection immediately after the proxy receives a connection 1403 from the remote host 1400. To prevent continuous cycles of reverse test connections, the proxy would then preferably perform name categorization in steps 1422, 1423, and 1424 before attempting to open a reverse test connection. Further, if the test connection is opened successfully, the proxy must then immediately close the connection, thus causing a remote Active Filtering proxy to exit before it can open a reverse test connection back to the current proxy. The preferred embodiment shown in Figure 17 depends upon administrators of remote networks continuing to use logical sequences when they assign names to their dialup addresses. Such sequential names are easiest to define and maintain, so it is in the interest of these administrators to use a sequential naming scheme. However, ISPs might also assign irregular names or name lengths to their node addresses. The only conceivable reason for making such assignments would be to permit their users to avoid Active Dialup detection, so such addresses can be manually added to the blacklist database if they are not appended by subsequent Active User filtering. The following alternative embodiments are more flexible with respect to detecting such irregular naming sequences, but have other side effects as noted. These embodiments could be used in step 1423 in place of, or in conjunction with, the preferred embodiment shown in Figure 17. In accordance with the alternative embodiment shown in Figure 22, the proxy can categorize a remote host as dialup or non-dialup based on the edit distance between the remote host's name and its neighbors' names. For example, a change of a "3" to a "7" involves an edit distance of one, as does insertion of a character, or deletion of a character. In conjunction with its failure to establish a reverse test connection 1418, a proxy can conclude that a low edit distance is evidence that the remote host name is part of a set of closely-related names consistent with a dialup name space. This method could be used to replace the method shown in Figure 17 (and referenced in Figure 16 step 1423) where names are closely-related, but not necessarily sequential. With respect to Fig. 22, steps 1500-1520 are as described in Figure 17 and provide a method of acquiring the neighboring host names for the remote host 1400. In step 1530, the proxy accesses the Dialup DB 1097 to acquire the threshold value to be used in the remaining steps. Possible threshold values are 1, 2, and 3, since an edit distance of 0 would indicate identity and thus not be useful and an edit distance greater than 3 is too broad and would result in miscategorization of non-dialup hosts as dialups. An edit distance of 1 would indicate a high degree of correlation (names would match only if they differed by one character), but would fail to match names such as "dial39" and "dial40". A threshold of 2 is probably optimal, even though it would improperly categorize rollover situations such as "dial99" to "diallOO". A threshold of 3 would address the aforementioned rollover problem, but would miscategorize, for example, a remote host "mail" surrounded by a sufficient number of hosts with names such as "main", "mail2", and "menu". In step 1531, the proxy scans each character of the neighbor name and compares it with the corresponding character in the remote host name. If the two characters are identical (step 1532), then the proxy advances the character pointer in the two names. In steps 1533, 1534, and 1535, the proxy determines if a character must be replaced, inserted, or deleted to make the neighbor name consistent with the remote host name. If so, it increments the edit distance 1536 for the neighbor name and continues. When the comparison is complete, the proxy then checks if the accumulated edit distance is less then or equal to the threshold read from the Dialup DB in step 1530. If so, it increments the match count 1538. The proxy then continues with the next name, as determined by step 1502. In another alternative embodiment, the host may be categorized as either dialup or non-dialup based on a correlation value between names and IP addresses. The correlation of a set of (x, y) values is a classic statistical problem. A low value (0.0) indicates no correlation, while a value approaching 1.0 indicates a high correlation. The x value in this case would be the node IP address (e.g., 107, 108, 109, etc.) and the y value would be some numeric representation of the host name (e.g., lCustl07, lCustlOδ, lCustl09, etc.). In this example, the correlation would be exactly 1.0 because the names are assigned sequentially, but a lower correlation might still be evidence of a dialup address. Still another alternative embodiment involves categorizing the remote host based on the ability to establish reverse test connections to its neighbors as well as the remote host itself, step 1418 (Fig. 15). If a sufficient number of neighboring addresses also do not permit reverse test connections, then it is reasonable to conclude that the remote host is a dialup. This method might be used by itself to replace the method shown in Figure 17, or may be combined with the method in Figure 17. With respect to Figure 23, the steps 1500-1520 are as described in Figure 17 and provide a means of stepping through each of the 20 nearest IP addresses for the remote host. In step 1550, the proxy attempts to connect to the neighbor x, using the same means described in step 1418. It checks the status for the connection in step 1551. If the connection is not successful 1552, as would be typical for a block of dialup addresses, the proxy increments the match count. However, if the proxy is able to establish a reverse connection to the neighbor x, it subtracts 2 from the match count 1553 and closes the test connection 1554. This weighting permits as many as three neighbors to accept reverse connections and still categorize the remote host as a dialup. The proxy then continues with the next name, as determined by step 1502. The disadvantage of this approach is that it maybe time-consuming to attempt a large number of reverse SMTP connections. However, it is less time consuming to perform this test in SMTP filtering software than it is to deal with the spam or junk mail after it is received on the organization's mail server. As described above, various conventional systems maintain static databases that list blocks of dialup addresses. However, the problem with these lists is that they are static (not dynamic). Consequently, a new block of addresses can be abused by dialup PCs numerous times before it goes on one of these lists. Further, if such a block of addresses is blockaded, and subsequently reused for a legitimate mailserver, then legitimate mail will be rejected because of the history of that IP address. An advantage of the Active Dialup filter of the present invention is that it is dynamic, and so categorizes the remote host at connection time based on non-response to a reverse SMTP connection and certain characteristics of the names of the remote host and its neighbors. Active Relay Testing In Figure 18, the proxy 1401 continues with Active Relay testing at step 1450 after successfully opening a reverse test connection to the remote host (step 1419, Figure 15). Active Relay testing is performed only for non-trusted hosts (as determined in step 1417) and if the reverse connection 1418 was successfully established. The Active Relay test characterizes the remote host with respect to its perceived acceptance of a reply to the supposed sender and whether the remote host is likely to be an open relay. The proxy 1401 performs Active Relay Testing by testing the validity of the MAIL From address on the reverse connection and implementing a relay test, such as the one shown in Figure 5. These tests are conducted while the remote host 1400 is connected to the proxy 1401 as a factor in determining whether to accept the remote host's message. If the remote host 1400 gives an indication that it will not accept a reply email message to the purported sender or that it will relay a test message from the proxy 1401, then that remote host is at an increased risk for relaying mail from someone else to the local MTA 1402. On the other hand, if the remote host indicates that it will accept a reply message for the sender and that it will not relay for the proxy, then the remote host probably is not an open relay. The proxy 1401 performs this test using the reverse SMTP connection 1418, then continues with the protocol transactions in steps 1454, 1456, 1458 and 1465. The test simply monitors the responses from the remote host, and does not actually send an email message to the remote host 1400. The local MTA 1402 is not involved in this test. At steps 1453-1467, the proxy server 1401 performs the active relay test. Steps 1453 through 1458 are preliminary steps required to progress to the relay test, while steps 1460 and 1467 provide the answers to the relay test. In step 1453, the proxy server 1401 reads the remote host's greeting 1452 from the open connection 1418. As indicated in item 1011 in Figure 2, when an MTA receives a connection to its SMTP port, it writes its system greeting to the connection to indicate that it is ready to receive mail. The proxy reads and discards each line of the greeting, handling multi-line greetings as described for Figure 2, since the greeting does not contain any useful information. If any read fails or if the first three characters of the greeting are not "220", the proxy exits from the relay testing sequence and continues with Active User testing of the supposed sender's mailhost in step 1901. In step 1454, if the greeting is received without errors, the proxy sends a HELO message to the remote host. The text of the message is a configurable string (read from the configuration database 1098, Fig.7), defined when the proxy is installed, and typically identifies the local host name. It should be noted that the HELO exchange is optional in most cases. However, some hosts require a HELO, which is therefore preferably included in the relay test. In step 1455, the remote host 1400 sends its reply to the HELO message. In step 1456, the proxy 1401 reads the HELO response sent in step 1455. If the read fails or if the first three characters of the greeting are not "250", the proxy exits from the relay testing sequence and continues with Active User testing of the sender's mailhost, step 1901. If the HELO reply is received without errors at step 1456, then the proxy issues a MAIL From message to the remote host. As noted above, the preferred embodiment uses the reserved name "reverse" to notify another Active Filtering proxy that this is a reverse test connection so that the remote proxy can avoid a connection loop. The address should identify the local domain name (for legal reasons). Thus, the message appears as MAIL From: <reverse@local.dom>. The remote host 1400 will then respond as shown in step 1457, typically with a "250" response (to indicate acceptance) or a "550" response (to indicate non-acceptance). As shown in step 1458, if the remote host 1400 replies with anything other than a "250" response, or does not respond, the proxy exits from the relay testing sequence and continues with Active User testing of the sender's mailhost, step 1901. If the remote host 1400 accepts the proxy's MAIL From message on the reverse connection 1418, then the proxy 1401 issues a RCPT message 1458 to the remote host on the reverse test connection. The proxy gives the complete MAIL From address of the supposed sender of the message ("mfaddr") as received previously in steps 1413 and 1414. This is equivalent to doing a real-time reply to the sender of the message, except that no data is transferred. In steps 1458 and 1460 the proxy determines if the remote host will accept the address of the supposed sender of the message. This may appear to be designed to determine the actual existence of the user "mfaddr" as is performed by the Active User test at step 1901 and there is some overlap if "mfaddr" actually exists at the remote host 1401. However, in the general case, this step is designed to determine if the remote host is configured to deliver (either locally or by relaying) email to the supposed sender. If at step 1460 the reply is "250" then the remote host will accept mail to this address, so the proxy continues with step 1462. Otherwise, if the reply is anything other than "250", the proxy sends an error response 1461 on the data connection 1403 and exits, thus closing the data connection 1403 and the reverse connection 1418. In the preferred embodiment, the proxy also writes a system log entry for the rejected message and adds the remote host's IP address to the blacklist database 1095 (Figure 7) before exiting. For example, smallhost.dom may be a customer of bigisp.dom and uses the "smart host" smtp.bigisp.dom to handle its mail forwarding and to receive mail when it is not online. Thus, if smtp.bigisp.dom connects with MAIL From: <someone@smallhost.dom> and accepts a reverse test connection, then smtp.bigisp.dom should accept the proxy's RCPT To: <someone@smallhost.dom> message on the reverse test connection. However, if smtp.bigisp.dom connects with MAIL From: <getrichquick@bulkmail.dom> and accepts a reverse test connection, but will not accept a reply to the MAIL From address, this is evidence that the message is forged. It should be noted that this is only for the situation where the remote host will accept a reverse test connection. If the network will not accept a reverse test connection because it has separate incoming and outgoing mail servers, then processing will follow the Active Dialup 1420 path as shown in Figure 15. At step 1462, the proxy 1401 attempts to find if the IP address of the remote host 1400 matches a non-relay entry in the Relay database 1096 (Fig. 7). This database lists blocks of addresses that the local organization must exchange email with, but which would fail the relay test. There might typically be between about 5-50 entries in this database, with each entry covering a block of addresses. These entries can be pre-defined by a site survey performed by each organization, preferably before installing the Active Filtering proxy server. For simplicity, the preferred embodiment of the Relay Database 1096 (as with other IP addresses listed in steps 1406 and 1413) expresses these addresses as a dotted-quad IP address, a forward slash "/", and a number of bits to be matched. Other embodiments may use other representations (hashed lists, dbm filed, or CAM) for performance reasons. If the IP address matches any non-relay entry, then the proxy 1401 bypasses any further relay and user testing, and proceeds with message transfer in step 1470. If the IP address of the remote host does not match any entry in the non-relay list, then it continues with the second part of Active Relay testing at step 1463. Before performing the relay test, the proxy compares the MAIL From domain with the connecting host name. A match at step 1463 occurs when the connecting host name and the MAIL From domain are identical, beginning at the end of the two strings, and comparing backwards over the last two nodes (i.e., periods) of the two domains. For example, if host "smtp. gamma. dom" connects with MAIL From "alpha@gamma.dom", the two domains match over the scope "gamma.dom". However, if host "smtp. gamma.dom" connects with MAIL From "alpha@beta.dom", the two domains do not match. If the two domains match in step 1463, the proxy checks the Relay database 1096 at step 1464 to determine if an administrator has configured the proxy to perform loose relay testing. With loose testing, the proxy permits mail from open relays if the MAIL From address matches the connecting host name. In this case, the relay test message 1465 is not necessary, so the proxy continues with transfer of the message in step 1470. If either of the domains do not match in step 1463 or the Relay database is configured for strict relay testing, the proxy issues a RCPT message 1465 identifying a configurable string, defined when the proxy is installed. The default includes the name "relayto" and the local domain name (for legal reasons), for example, RCPT To: <relayto@local.dom>. The configurable recipient address may be any syntactically correct address. Even though a message is not sent the RCPT address is preferably not a real user address in order to avoid address mining by spam site administrators. The remote host 1400 will then respond as shown in step 1466, typically with a "250" response (to indicate that it is willing to relay) or "550" (to indicate that it will not relay). In step 1467, the proxy 1401 determines the status of the reply message from the remote host. If the reply is "250", then the remote host will apparently relay for the proxy, so the proxy rejects the message as indicated in step 1468 and exits, thus closing all connections. In the preferred embodiment, the proxy also writes a system log entry for the rejected message and appends the IP address to the blacklist database (Figure 7, item 1095). If the reply to the RCPT message 1462 is anything other than "250", then this indicates that the remote host is not an open relay, and so the proxy continues with reception of the message at step 1470. Thus, the proxy sends two RCPT messages (1458 and 1465) to the remote host. The remote host must respond with a "250" to message 1458 and with anything other than "250" to message 1465. This establishes that the remote host is responsible for the MAIL From address received in step 1414 but is not an open relay that will accept any RCPT address. Other combinations are described in the following paragraphs. If the proxy responds with a "250" to both RCPT messages, then it is not possible to tell if the MAIL From address 1413 is legitimate or not. In the previous example, a legitimate message from <someone@smallhost.dom> may be sent via an open relay smarthost smtp.bigisp.dom. Alternately, the message could well be forged, since the remote host is an open relay. For example, referring to step 1064 of Figure 4, the spammer forged the MAIL From address "good@relay.dom". Thus, when the proxy receives "250" responses to both messages, the preferred embodiment of the Active Relay method is to reject the message and log its rejection. If a subsequent review of rejected messages shows a legitimate address, the administrator ofan Active Filtering proxy can then add the individual address to the whitelist database (Figure 7, item 1094) or can bypass relay testing for smtp.bigisp.dom by defining it as a non-relay in the Relay database 1096 (Fig. 7). The remote host may also respond with a "550" (for example) to message 1458 and a "250" to message 1465. Some hosts will permit promiscuous relaying but reject any non-existent local addresses. In this case the proxy rejects the email message. The remote host may respond with a "550" (for example) to both RCPT messages. In this case the remote host itself is probably not an open relay. The bad address is probably either forged by a user at the remote host or there exists a multi-hop relay path through some other host that is trusted by the remote host 1400. The administrator may manually edit the relay database to add an IP address if a review of log entries shows that the remote host is an authorized "smart host", i.e., a host authorized to handle the local network's outgoing email. In addition, certain MTA programs give a "250" reply to the RCPT message 1465, but then discard the message later on. These may be configured as trusted or as non-relay. The active relay method permits automatic rejection of all email sent from a user at an open relay host or relayed by an open relay host. However, in some cases it is necessary to override this behavior, for business or other reasons. For example, with respect to the bigisp example given above, the administrator of the Active Filtering proxy can configure the proxy to permit email sent from smallhost.dom and relayed by bigisp. dom by any one of the following actions: (1) defining bigisp.dom as a trusted domain, (2) adding a whitelist entry for the specific address "user@smallhost.dom", (3) or adding a non-relay entry for bigisp.dom, even though it is an open relay. A pre-installation site survey can anticipate such problems by reviewing system logs and testing all hosts that have recently connected to the local organization for relaying. Any open relays can then be configured in the respective databases before the Active Filter proxy is installed. No special actions are required for hosts that do not relay or for hosts that do not routinely send mail to the local organization. With reference again to Figure 18, if the Active Relay method determines the remote host 1400 to be an open relay, it sends an error message 1468 to the remote host, appends the IP address of the open relay to the blacklist file, writes an entry to the system log, and exits without transferring the message to the local MTA. Otherwise, if the remote host is not an open relay, the proxy continues with message transfer at step 1470. It is noted that relay testing is more accurate than user testing, so that user testing may be skipped. The preferred embodiment of the Active Relay method performs testing in the order shown in Figure 18. In particular, this ordering avoids the relay test sequence if the remote host is configured as a non-relay (step 1462) or the domains match and loose relay testing is configured (steps 1463 and 1464). However, alternative embodiments may provide for other orders of testing. The Active Relay method makes decisions based upon responses to the proxy's RCPT requests. However, certain MTA products issue a "250" to these RCPT requests but defer enforcement until it receives a DATA message or the closing "." indicating the end of the message. The proxy can be adapted to recognize these products by looking for certain characteristic data (e.g., the product name) in the greeting message 1452. In this event, the proxy can monitor subsequent responses for these products, such as a response other than "354" to a DATA message. Alternatively, the proxy can be extended to accept certain remote hosts based on product type. Active User Testing The Active User method illustrated in Figure 19 determines if the MAIL From address 1413 is acceptable to a mailhost 1900 configured to receive email for the MAIL From domain. By convention, this mailhost is either a Mail Exchange (MX) host or the host identified in the MAIL From message. This method uses the same SMTP messages described for the Active Relay method (Figure 18, steps 1454-1458), but in most cases the proxy accesses a different mailhost than the remote host 1400. While the Active Relay test is concerned with determining if the remote host 1400 is at risk for sending relayed or forged email, the Active User test accesses the mailhost 1900 responsible for receiving email for the MAIL From domain to determine if it will accept email for that address. If it does not accept the MAIL From address, then this indicates that the MAIL From address is probably forged and does not exist on that network. For example, assume the remote domain remote.dom has two mail servers, out.remote.dom for sending outgoing email and mx.remote.dom for receiving incoming email. If the proxy 1401 receives a connection 1403 from out.remote.dom, the proxy will be unable to establish a reverse test connection 1418 to that particular host because it is not configured to accept incoming SMTP connections. Assuming that the host names surrounding out.remote.dom do not appear to be dialups, it remains for the Active User method to attempt to validate the MAIL From address. In this case the proxy 1401 would find the MX host mx.remote.dom and query that host as to the validity of the MAIL From address. This Active User method is not completely accurate by itself, but does provide an additional level of testing when the proxy cannot query the remote host 1400 via the reverse test connection. Consequently, this method is used following Active Dialup testing (where there is no reverse test connection 1418) and after encountering errors in Active Relay testing. The reason this method is not highly reliable by itself is that some large networks (AOL.COM, YAHOO.COM, MSN.COM) accept (that is, give a "250" response to) all RCPT messages identifying properly-formatted addresses on their respective networks. This is done both for performance reasons and to prevent outsiders from verifying or collecting addresses on that network by sending many different possible addresses and monitoring the responses. Still, enough networks do truthfully respond to RCPT messages to make the Active User method useful as a supplement to the Active Dialup and Active Relay methods. With respect to Figure 19, the Active Filtering proxy 1401 begins operation at step 1901 subsequent to either Active Dialup or Active Relay testing. In step 1902, the proxy identifies a mailhost 1900 responsible for receiving mail to the MAIL From address 1413. The proxy searches the Domain Name System (DNS) information for the MAIL From domain for records identifying Mail Exchange (MX) hosts for that domain. MX records include a host name and priority value, and by convention the lowest priority value identifies the MX host that should be tried first. In the preferred embodiment, the proxy uses resolver library routines such as the DNS BIND res_init() and res_query() functions to access the MX records, however, other methods may be used to access the name server. If no MX host is found, then the proxy uses the MAIL From address (that is, the host name to the right of the "@" character in the MAIL From address) as the mailhost. In step 1903, the proxy attempts to connect to the mail server identified in step 1902. This follows the same mechanisms described in step 1418, except that the TCP connection is to the identified mailhost 1900 rather than to the remote host 1400. If the connection is successful in step 1904, the proxy waits at step 1906 for the system greeting 1905 from the mailhost. Otherwise, if the connection is unsuccessful, the preferred embodiment of the proxy simply proceeds to step 1470 for message transfer to the MTA. In alternative embodiments, the proxy might successively check for lower-priority MX hosts if the highest priority host is not available. It is noted that for some networks where the same host handles both incoming and outgoing email, the mailhost 1900 may be the same (that is, have the same IP address) as the remote host 1400. In this case, the proxy simply makes a second test connection to the same host without regard to having previously tested the MAIL From address at this host. Steps 1905-1912 follow steps 1452-1459 in the Active Relay test in Figure 18, respectively. After receiving the system greeting, the proxy issues a HELO message in step 1907. Subsequently, after receiving the HELO reply, it sends a MAIL From message in step 1909 with the reserved Active Filtering test address. And finally, it sends a RCPT message in step 1911 with the MAIL From address received in step 1414 (Figure 15). In step 1913 the proxy inspects the mailhost's reply to the RCPT message. If the reply is "250", the proxy continues with message reception at step 1470. If the domain mailhost 1900 gives any reply other than "250", then in step 1914 the proxy 1401 sends an error reply to the remote host 1400 on connection 1403, closes data connection 1403 and test connection 1903, logs the rejection, adds the IP address of the remote host 1400 to the blacklist database, and exits. Preferably, when appending an IP address to the blacklist database 1095, the proxy adds a 4-byte IP address, e.g., 192.168.200.45, along with some number of bits to be matched. Typically the number of bits is 24, so that any subsequent connection by any host in the range 192.168.200.0 through 192.168.200.255 will be rejected by the blacklist mechanism. This takes into account that Class C addresses are normally assigned to organizations in multiples of 256, so subsequent connections in the 192.168.200.x range are normally owned by the same irresponsible organization, so it makes sense to block all of them. However, if the ownership is subsequently determined to be something more or less than a single class C, then the blacklist file can be manually edited to block one or more hosts. Other than the trusted database, the preferred embodiment does not require any special databases to control the Active User test mechanism. In an alternative embodiment, a database prevents unnecessarily testing networks such as AOL.COM that automatically respond with a "250" to RCPT addresses. Connect to MTA All message rejections to this point have not involved any storage (other than log entries) being allocated on either the proxy or the MTA host computer. If all tests have been successful, then Figure 20 shows how the proxy 1401 connects to the MTA and transfers the initial SMTP messages required to set up the transfer of the message to the MTA. In step 1470 the proxy connects to the MTA using the same method described for the reverse test connection 1418. In summation, the proxy connects to the MTA for messages with any of the following characteristics: connection from a trusted domain (step 1417); whitelisted MAIL From address (step 1417); or email from a user with an account at a non-dialup, non-relay host (step 1467). In addition, subject to the validity of the user address as determined in step 1913, the proxy will permit mail from hosts where the reverse connection fails, but host is configured as non-dialup (step 1421); reverse connection fails, but host is not detected as a dialup (step 1424); reverse connection succeeds, but relay test is inconclusive (steps 1454, 1456, 1458); reverse connection succeeds, but host is configured as non-relay (step 1462) or reverse connection succeeds, MAIL From address matches connecting host, and the proxy is configured for loose relay testing (steps 1463, 1464). At steps 1470-1475 of Fig. 20, the proxy 1401 connects to the local MTA 1402, sends the accumulated information received so far from the remote host, and starts to transfer data between the remote 1400 and the MTA 1402. The proxy opens the TCP connection 1470 and awaits a greeting 1471 from the MTA. When it receives the greeting, it sends the HELO message 1472 to the MTA that it received from the remote host 1400 in step 1430. If no HELO message was received from the remote host, then the proxy does not send the message 1472. The proxy then awaits the response 1473 from the MTA, and sends the MAIL From message 1474 that it received from the remote host 1400 in step 1433. The proxy 1401 then awaits the MTA's response 1475 from the MAIL From message, and writes that response immediately to the remote host 1400. In steps 1480 and 1481, the proxy simply receives a RCPT message from the remote host 1400 and passes it transparently to the MTA 1402. In steps 1482 and 1483, it receives the SMTP reply from the MTA and passes it transparently to the remote host. The remote host may send multiple of these RCPT messages, each of which is handled in the same way. In an alternative preferred embodiment, the proxy keeps track of the number of recipients (both those accepted by the MTA and those rejected by the MTA) and issues an error message when the remote host exceeds the maximum number of recipients configured in the configuration database 1098. Message Transfer and Bcc Filtering At this point, the proxy 1401 is operating in a transparent pass-through mode. Prior to step 1475, the proxy operates at the application level, where it handles the SMTP messages on behalf of the local MTA. Beginning with step 1475, the proxy operates in a filtering mode where it simply transfers data between the remote host 1400 and the MTA 1402, with limited filtering. Figure 21 shows the processing steps involved in transferring the actual email message from the remote host 1400 to the MTA 1402. Except for the limited filtering performed in steps 1491 and 1492, the proxy transparently transfers the SMTP DATA command 1484, message header lines 1488, message body lines 1490, 1493, and closing protocol lines 1495 from the remote host 1400 to the MTA 1402. By convention, the message header is defined as all lines of the message down to, but not including, the first empty line. The proxy also transparently transfers SMTP replies from the MTA to the remote host (steps 1486 and 1497). As is consistent with SMTP, the lines of the message header and message body are not individually acknowledged by the MTA. The proxy may integrate any conventional filtering method, such as one that prevents Blind Carbon Copy (Bcc) messages. Such mail is characterized with a local domain address in one of the RCPT To: messages 1480, but without a reference to the local domain in the header lines 1488 of the message. This can either be legitimate and intentional (as when a sender defines a Bcc address in their mail reader) but is generally abused by junk mailers in order to save the processing time required to reprocess the message header for each recipient. In step 1491, the proxy 1401 checks To, Cc, and associated continuation lines for a local domain address. The filter permits Bcc messages only for trusted addresses. If it has not found a local To or Cc address when it reaches the last line of the message header, it writes an error message to the remote host, closes the data connection 1403, logs the event, appends the IP address of the remote host to the blacklist 1406, and exits, step 1492. This appears to the MTA 1402 as an interrupted TCP connection. The MTA typically logs this event to the system logfile, but does not send the partial message to any local users. In steps 1495-1498, the proxy 1401 closes the connection 1403 from the remote host and the connection 1470 to the local MTA when it senses that either connection has been terminated. Typically, the remote host closes the connection by sending a "QUIT" message 1495 to the MTA 1402. The MTA then writes a status response 1497 and closes the connection 1470. The proxy 1401 senses the closed connection and closes the connection 1403 from the remote host 1400. This ends the transfer of the single SMTP email message associated with the current instance of the proxy 1401, and the proxy 1401 exits. In the preferred embodiment, multiple messages from multiple remote hosts are handled by relying on the proxy server's operating system 1090 (Fig. 7) to run multiple instances of the proxy process, one for each message. However, other implementations are consistent with this invention such as, for example, a multi-threaded proxy server process that handles multiple messages. The preferred embodiment has been described as having IP filter testing 1406, followed by Active Dialup testing 1420, Active Relay testing 1450 and Active User testing 1900. However, it should be appreciated that these tests may be implemented in various other orders. In addition, each of these tests have individual uses, and need not be used in connection with one or more of the other tests. The foregoing descriptions and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of different manners and is not limited by the preferred embodiment. Numerous applications of the present invention will readily occur to those skilled in the art. For example, though the preferred embodiment of the invention is implemented on a global network, such as the Internet, it may also be used, for instance, in an isolated intranetwork or using closed networking protocols such as Novell Netware. In addition, in the event that it becomes illegal to connect to any mail server except to transfer legitimate email, a message may be transferred to the MAIL From address saying "We have received your SMTP connection and are considering whether to accept your email message." The preferred embodiment tests for relaying only for non-trusted hosts who attempt to deliver mail to the local MTA. While the preferred embodiment uses Internet standard protocols such as IPv4, DNS, TCP, and SMTP, the invention may also be used with other networking protocols and network architectures, such as, for instance, IP version 6 (IPv6) or X.500 name services, or protocols not yet developed. Further, the invention maybe used with other backbone MTA-to-MTA protocols such as Extended SMTP (ESMTP), the X-400 Message Handling System (MHS) or client-to-mailhost protocols such as POP or EVIAP when the Active Filtering functions are not performed on the backbone. Further yet, the invention may be used with various cryptographic architectures such as Secure Socket Layer (SSL), IP Security (IPSec), S/MMIE or OpenPGP standards, although spammers are unlikely to use any protocols involving traceable encryption keys. The Active Filtering methods described in this application can be integrated with other suitable devices and/or methods to provide additional capabilities. The proxy can also be configured with additional databases not described in this application to provide further controls or increased performance. For example, the preferred embodiment of the proxy does not provide for appending hosts determined to be non- relay or non-dialup to these respective databases. However, subject to performance requirements, other embodiments of this proxy might perform caching of tested addresses so as to avoid unnecessary re-testing. Per-Recipient Whitelisting and Quarantining Two additional features of the preferred embodiment of the invention, per-recipient whitelisting and quarantining, can be used by individual users of the network to manage their own incoming email and to retrieve messages that were rejected by Active Filtering. This embodiment uses the same Active Filtering (that is, Active Dialup 1420, Active Relay 1450, and Active User 1900) tests as described with reference to Figures 7-23. Unlike the system described in Figures 7-23, which enforces access decisions during processing of the MAIL From message, these two additional features require that the proxy defer enforcement of Active Filtering decisions to RCPT time. This is because the proxy does not know the recipients ofan email message at MAIL From time. It is only when the sending MTA 1400 identifies each intended recipient (that is, by sending an RCPT message) that the proxy 1401 can access that recipient's whitelist and determine how to process the message for that recipient. For example, one recipient may have a whitelist entry to receive mail from a particular sender, while another recipient may choose to discard mail from that sender. It should be apparent, however, that per-recipient whitelisting and quarantining can be performed in any suitable manner and at any suitable stage of mail processing subsequent to RCPT. Figure 24 illustrates the additional databases used by this preferred embodiment. With respect to Figure 7, Figure 24 adds an optional Per-recipient Whitelist Database 1600 for each recipient and a Quarantine Database 1610 for storage of rejected messages. These databases are accessed by RCPT message processing, so a block for RCPT message processing was also added in Figure 24. The per-recipient filtering can be flexibly configured by the recipient. First, this filtering allows the recipient to have a whitelist containing a single "@" that will match any sender's address and thus totally override Active Filtering for that recipient. Second, the recipient can have a whitelist that enumerates certain sending domains/addresses that will override Active Filtering. In this case, mail from one of these senders that is detected as a spam risk by Active Filtering will be permitted for that recipient and blocked for all other recipients not having a matching whitelist. Third, other recipients can operate without a per-recipient whitelist, in which case the Active Filtering decisions will block mail in accordance with Figures 1-23. The Per-RCPT Whitelist Database 1600 consists of a collection of whitelist files, where each file pertains only to a particular recipient. The whitelist files 1600 can be configured by a system administrator, or with suitable access controls, by the recipients themselves, so that one recipient can receive an Active Filtered message from a particular sender while the message is blocked for another recipient. The per- recipient whitelist files 1600 are provided in addition to the system whitelist 1094. Accordingly, a particular recipient will receive a message if the sender is listed in either the system whitelist 1094 or that recipient's whitelist 1600. In addition, the filter can quarantine messages that would otherwise be rejected. A quarantined message is received by the proxy and saved to disk storage, where it can subsequently be reviewed and forwarded by either the recipient or an email administrator. The proxy can be configured to separately enable quarantining for Active Relay, Active Dialup, and/or Active User rejections. For example, a proxy can be configured to quarantine messages that fail Active Relay testing and reject messages that fail Active Dialup and/or Active User tests. Per-recipient whitelisting and quarantining are performed separately for each recipient, that is, for each RCPT address. Thus, in general, an email message with multiple recipients may be whitelisted for some recipients and quarantined (or rejected outright) for the remaining recipients. Of course, the recipient whitelist need not be defined and/or the quarantining need not be configured by the system administrator. This would effectively remove these features from the filter proxy. The present preferred embodiment (shown in Figures 7-23) provides a uniform level of junk mail protection for all recipients. That is, the Active Filtering Proxy 1401 determines if the message is unacceptable (i.e., the remote host is an open relay, the remote host is a dialup, or the message has a forged MAIL From address), then either accepts or rejects the message for all recipients. This is best suited for large organizations such as Government agencies or commercial businesses that have a consistent policy regarding junk mail and that have decided to block all potentially- dangerous email. However, most ISPs and some businesses have users with widely varying expectations regarding junk mail filtering. Some users may want the proxy to block all potentially dangerous mail, while other users will tolerate significant amounts of spam but do not want to have a single message inadvertently blocked. For this situation, the Active Filtering tests (Active Relay 1450, Active Dialup 1420, and Active User 1900) are used in conjunction with per-recipient whitelists to permit individual users to receive mail that might otherwise be rejected by the Active Filtering tests and a quarantine mechanism to save email that is rejected by Active Filtering and does not match a recipient whitelist. Thus, the whitelist 1600 is flexible to provide support for a large organization having a single junk mail policy or for an ISP that allows the individual recipients to define their own respective filtering policies. The recipient whitelists 1600 are consulted only if a message would be intercepted by one of these Active Filtering tests. Otherwise, if the remote host is not an open relay, the remote host is not a dialup, and the address does not appear to be forged, the proxy 1401 passes the message to the MTA 1402 without considering user whitelists 1600 and without saving the message to the quarantine database 1610. If a particular recipient(s) does not have a recipient whitelist 1600, then the proxy will deliver the message only if it successfully passes all of the Active Filtering tests. If an email message fails an Active Filtering test, is not whitelisted by all recipients, and message quarantining is configured in the Configuration Database 1098 (Figure 24), then the email message is quarantined for the recipients that do not have a whitelist that matches the sender's address. In accordance with the preferred embodiment of Fig. 24, each recipient whitelist 1600 is maintained in a separate file, located in the directory identified in the configuration database 1098, and having a name that identifies the recipient's email address. For example, if a proxy 1401 is an MX host for the two domains escom.com and foo.net, the recipient whitelist directory might have the recipient whitelist database files 1600 of Table 5. alice@escom.com asmith@foo.net bart@escom.com cap@escom.com postmaster@escom.com postmaster@foo.net psmith@foo.net rjones@foo.net Table 5 - Sample Whitelist Database 1600 Each of the files of Table 5 contains a sequence of one or more substring patterns for the recipient identified in the file name. If a user has multiple addresses (e.g., alice@escom.com and asmith@foo.net), then there is preferably a separate file for each address. Substring patterns are used to identify senders (MAIL From addresses) that are permitted to override Active Filtering decisions and thus send mail to the recipient. For example, the file alice@escom.com might contain @somedom.dom, @elsewhere.net andjane@doe.com. Thus, MAIL From: <george@somedom.dom> via an open relay to alice@escom.com would be permitted because the pattern "@somedom.com" is a substring of the MAIL From address. In addition, mail from jane@doe.com would also be accepted, though mail fromjohn@doe.com would not. The proxy 1401 also has a system- wide whitelist file 1094 that pertains to all recipients. As previously discussed with reference to Figure 15, the MAIL From address is whitelisted only if it is an exact match of an entry in the whitelist database 1094. However, the system whitelist 1094 and recipient whitelist 1600 entries are preferably tested as substrings of the MAIL From address to determine if it is a match. It should be noted that the recipient whitelists 1600 do not override blacklisted IP addresses. If a remote host is blacklisted, as shown in Figure 14, the proxy closes the connection from the remote host without proceeding to the exchange of MAIL From and RCPT To messages. If a message is rejected by Active Filtering and the proxy is configured for quarantining, the proxy will save the email message to a separate file in the quarantine directory. The quarantine directory is specified in the Configuration Database 1098, for example, /var/spool/asmtp/QD. The proxy assigns a name for each quarantine message file, such as including the characters "qf ', the numeric month, day, hour and minute, and the process ID for the proxy process. For example, the complete pathname for a quarantine file might be: /var/spool/asmtp/QD/qfO3051459-21481. Each quarantine file contains the remote host's name and IP address, the MAIL From address, at least one RCPT To address, a DATA line, and the text of the message as received from the remote host. For example, the first few lines of a quarantine file might contain: connect host=mercury.somewhere.dorn/l 92.168.100.10 MAIL From: <sender@somewhere.dom> RCPT To: <bob@local.dom> RCPT To: <carol@local.dom> DATA Received: from remote.dom (host.remote.dom [192.168.255.255]... The proxy creates quarantine files when required for a message matching the quarantine criteria. The quarantine files remain in the quarantine directory until they are manually removed by an administrator, retrieved by all recipients, or automatically removed by an operating system command (such as the UNIX cron command) that finds and deletes files of a certain age. For example, an administrator may periodically review the quarantined messages in order to delete those messages that contain spam or objectionable material and forward (and then delete) those messages which are relevant to the organization's business. The proxy itself does not provide any capabilities for managing quarantine files, but supplementary tools permit administrators and users to forward quarantined messages from storage on the proxy host to the user's mailbox on the MTA. For example, an administrator can run a utility program (qadmin) on the proxy host to forward a quarantined message to the MTA, where it will be delivered to the recipients listed in the quarantine file. A network user can run a quarantine client (qc) program to list messages for that recipient, and to forward selected messages for that recipient to the user's mailbox on the MTA. Not all rejected mail is quarantined. The Configuration Database 1098 contains settings to separately control quarantining of Active Relay rejections, Active Dialup rejections, and Active User rejections. Thus, if an organization has a policy of not permitting any SMTP Direct email, then the proxy can be configured to reject Active Dialup messages without quarantining, while quarantining messages rejected by the Active Relay and Active User tests. One disadvantage of quarantining is that it requires disk storage on the proxy server host. When a message is quarantined, the proxy makes a local copy of the message on the proxy's file system. To prevent the proxy from running out of disk space, administrators must provide a large pool of disk storage, must periodically monitor disk usage, and must provide a means for removing quarantined messages after some site-defined quarantine period. One possible threat would be for an attacker to attempt to fill all the available disk storage by repeatedly sending large messages that would be quarantined by the proxy. This threat is reduced somewhat by automatic blacklisting of a remote host after it transfers the first message, but the attack could be modified to relay a large message from many different open relays, for instance. A related disadvantage of quarantining is that if the quarantined message has malicious content, the message may be retrieved by a user who will further spread the virus or malicious content. This risk can be ameliorated somewhat by using virus filtering tools to scan the quarantine directory for malicious content. However, these disadvantages are generally considered to be outweighed by the advantages of having copies of email rejected by the Active Filtering tests. One advantage of quarantining rejected email is that it provides the administrator with a ready database of all collected junk mail for potentially pursuing legal action against high-profile spammers. The administrator does not have to retrieve this evidence from individual users in order to pursue legal action. The operation of the preferred embodiment of Figure 24 will now be discussed with reference to the flow charts of Figures 25-28. The same Active Filtering methods (Active Relay 1450-1465, Active Dialup 1420-1423, and Active User 1901- 1913 tests) are used as described with reference to Figures 16, 18 and 19. However, enforcement of the Active Relay, Active Dialup, or Active User test results are deferred until each RCPT message is received. The RCPT message identifies each intended recipient, which permits the proxy to find the whitelist 1600 (if it exists) to be used for that recipient. Consequently, for both per-recipient whitelist and message quarantining this alternate embodiment involves changes to the proxy 1401 for MAIL From (Figure 25), RCPT To (Figures 26-27) and DATA (Figure 28) message processing. In the preferred embodiment, the proxy provides mode flags that can be set by an administrator so that the proxy will perform only the selected Active Filtering tests. If a particular mode flag (e.g., modedial) is not set, the proxy will not perform that particular test for any sender and, thus, will not reject any message for that reason. If all of the mode flags are set, the proxy will preferably perform the tests in the order shown in Figure 25. In other alternative preferred embodiments, the proxy can perform the Active Filtering tests in other orders. In Figure 25, steps 1413 - 1419, the proxy 1401 attempts to open a reverse test connection 1418 to the remote host 1400. The proxy 1401 then performs Active Relay testing (1450-1465) if the connection is successfully opened, or performs Active Dialup testing (1420-1423) otherwise. Finally, the proxy 1401 performs Active User testing (1901-1913) if the results of Active User or Active Dialup testing are inconclusive (e.g., protocol failure), or if the remote host is not identified as a dialup 1602. The proxy 1401, however, does not immediately enforce the Active Filtering test results by rejecting any message that fails the relevant Active Filtering tests. Instead, the proxy 1401 checks for an open relay 1601, dialup 1602, or forged address 1603, and then sets a reject flag in step 1604 if it finds any of these conditions. The proxy also reads quarantine flags (step 1605) from the Configuration Database 1098. There is a separate quarantine flag for each of the three aspects of Active Filtering, thus permitting Active Relay rejections to be quarantined, for example, while rejecting outright Active Dialup and Active User failures. Finally, the proxy then sends the MATT, response message to the remote host in step 1606. Figures 26 and 27 illustrate the steps involved for the proxy 1401 to process a single RCPT message from the remote host. The remote host may send multiple (n) RCPT messages, up to the maximum number of recipients permitted by the proxy. This is indicated symbolically as n=l-maxrcpt, where the maximum number of recipients, maxrcpt, is defined in the configuration database 1098. In step 1630, the remote host 1400 sends the RCPT message with the email address of the intended recipient, shown symbolically as <rcpt-n>. At step 1631, the proxy determines if the message is trusted, either (a) because the remote host matches an entry in the trusted database 1093 or (b) because the MAIL From address matches an entry in the system whitelist 1094. In either case, the proxy proceeds to transfer the RCPT message to the MTA, beginning at step 1637. If the message is not trusted, the proxy determines if it has set the reject flag 1604 to indicate an open relay, dialup, or forged user address. If the flag is not set, then Active Filtering found no problems, and so the proxy proceeds to step 1637 with transfer of the RCPT message. Consequently, as a result of steps 1631 and 1632, any remaining messages are untrusted and were flagged by Active Filtering as a junk mail risk. These are messages that would have been rejected by the system in accordance with the Active Dialup, Relay and User tests, as shown in Figure 7. At step 1633 the proxy attempts to open the recipient's whitelist file 1600. This consists of getting the recipient whitelist directory from the configuration database 1098, appending a "/" character to separate the directory and filename, then appending the RCPT address. For example, if the recipient whitelist directory is /var/spool/asmtp/UW (where UW is User Whitelists) and the recipient is "asmith@foo.net", then the recipient whitelist file is /var/spool/asmtp/UW/asmith@foo.net. The recipient address is used as the name of the whitelist file because it is unique for each possible recipient address and because it simplifies file management. While this scheme does not permit grouping of multiple addresses with a single profile, it is understood that a separate table lookup mechanism could be used to map multiple recipient addresses to a single filename. At step 1634 the proxy checks the return status from the file open. The most likely reason for a failure is that the recipient does not have a recipient whitelist file 1600. A whitelist file is not required for every recipient, and the whitelist file is created at the option of the recipient or system administrator. Users without whitelists receive email that is accepted by the Active Dialup, Relay and User tests of the Active Filtering proxy. This includes mail from trusted sources and non-relay, non-dialup mail from legitimate addresses. If the recipient whitelist file 1600 is not available, the proxy proceeds to step 1650 where it checks the quarantine flags to determine whether to reject the message for this recipient or to add the recipient to a quarantine file. If the whitelist open was successful, at step 1635 the proxy reads entries in the whitelist file to determine if any whitelist entry is a substring of the MAIL From address. For example, if the MAIL From address was "george@somedom.dom" then any of the following patterns would match that address: @, george@, george, @somedom.dom, somedom.dom, or george@somedom.dom. The "@" pattern is a special case that will match any MAIL From address, because the proxy requires all MAIL From addresses to have an "@". The "@somedom.dom" pattern will allow the recipient to receive mail from any user at somedom.dom, while the "george@somedom.dom" pattern matches only the specific sender. If any pattern matches the MAIL From address, then the proxy transfers the RCPT message to the MTA beginning at step 1637. Otherwise, the proxy proceeds to step 1650 to determine whether to reject or quarantine the message for that recipient, logs the RCPT status (not shown), issues an appropriate status response, and waits for further input from the remote host. At step 1637, the proxy checks if this recipient is the first authorized recipient for this message. If so, the proxy connects to the MTA as shown in step 1640, sends the HELO message received earlier from the remote host, and sends the MAIL From transaction received earlier in step 1413 (Figure 24). If any error occurs (not shown), the proxy closes the data connection 1403 from the remote host, logs the status, and exits. The proxy sends the current RCPT message to the MTA in step 1646, waits for the MTA response, and sends the MTA response to the remote host in step 1648. Providing an actual MTA response to the remote host is important because the proxy does not know which users actually have mailboxes on the local MTA. The proxy knows if a recipient has a whitelist, but all users do not necessarily have whitelist files. Consequently, the proxy may accept a message for a recipient that does not have a mailbox on the MTA. In this event, the proxy should return the MTA's error message to the remote host. Continuing with steps 1650 and 1651 on Figure 27, the proxy checks the quarantine flags (from step 1605, Figure 25) to determine if the email message is to be rejected or quarantined for the current recipient. The proxy performs this test only after determining that the message is not trusted (step 1631), that the reject flag was set by Active Filtering (step 1632), and that the current recipient does not have a whitelist entry that matches the sender (steps 1632-1635). The quarantine flags are stored in the Configuration Database 1098 and retrieved after determining that the message failed an Active Filtering test in Figure 25. Individual flags are provided for each of the Active Filtering tests so that individual types of rejections can be independently enabled or disabled for quarantining. These flags permit organizations to configure the proxy to quarantine some types of risky messages (e.g., Active Relay failures) while rejecting outright other types of messages (e.g., Active Dialup and Active User failures), according to the local organization's policies. While quarantine flags are defined only for the various Active Filtering modes, other embodiments might provide additional flags for various conventional spam- filtering rejections, such as a non-existent MAIL From domain. A message is quarantined for a recipient if the quarantine flag is set for the corresponding Active Filtering rejection. For example, if the remote host fails the Active Relay test and the Active Relay quarantine flag is set, the proxy will perform quarantine processing beginning at step 1652. Otherwise, if the corresponding flag is not set, the proxy rejects the message for this recipient by sending a 550 response at step 1655. At step 1652 the proxy checks whether the quarantine file was opened for a previous recipient. If not, the proxy attempts to create a new quarantine file for the message at step 1653. This involves retrieving the quarantine directory name from the configuration file 1098 and appending a unique name for the file within that directory. The unique name is constructed by concatenating the numeric month, date, hour and minute and the numeric ID of the proxy process along with other fixed characters suitable for identifying a quarantine file. For example, if the quarantine directory is "/var/spool/asmtp/QD", the current date and time is March 5, 14:15 and the proxy's process ID is 15113, the pathname for the quarantine file would be: /var/spool/asmtp/QD/qf03051415-15113. At step 1654 the proxy checks the return status from the file creation request. If the proxy cannot create the file, for example, because it already exists or because of exceptions such as insufficient disk space or inadequate access rights, the proxy sends a 550 rejection to the remote host for the current recipient. If the quarantine file creation is successful, the proxy appends SMTP control information, including the remote host's name and IP address, and the MAIL From address at step 1656. Steps 1653-1656 are preferably performed only for the first recipient to be quarantined and not for successive recipients. The first recipient to be quarantined may or may not be the first recipient. For example, if the first recipient has a whitelist for the current sender then the message would be accepted for that recipient. If the second recipient then did not have a matching whitelist for the sender, the quarantine file would then be opened for the second recipient. At step 1657 the proxy appends the current recipient address. This occurs for each recipient that is not whitelisted or accepted for some other reason. For example, if a message has ten recipients, four of which are whitelisted, then the remaining six recipients would be appended to the quarantine file. The proxy then sends a "250" response to the remote host for the current recipient. In summary, the proxy sends a "250" response at step 1658 if the message is quarantined for this recipient, or a "550" response at step 1655 if the message is not quarantined. This might occur if the quarantine flag is not set (step 1651) or if the quarantine file could not be created (step 1654). This completes processing of a single RCPT transaction 1630 from the remote host 1400 to the proxy 1401. The recipient may have been accepted or rejected by the MTA at step 1648; rejected by the proxy at step 1655; or accepted for quarantining by the proxy at step 1658. After sending the MTA's RCPT response to the remote host, the proxy waits for further input in accordance with the SMTP protocol. This may include additional RCPT transactions (up to maxrcpt), a DATA message, or a QUIT message. The maxrcpt value is retrieved from the Configuration Database 1098 as part of overall initialization of the proxy. If the remote host attempts to send more than maxrcpt RCPT messages, the proxy rejects the additional messages with a suitable error status (not shown). In accordance with the SMTP protocol, a sending MTA will send a DATA message when it has successfully transferred at least one RCPT, or a QUIT message if no RCPTs were accepted by the receiving MTA. Figure 28 shows the steps involved in processing the DATA transaction and subsequent text of the message (including the message header, body, and attachments, if present). The addition of per-recipient whitelisting and quarantining in this alternative preferred embodiment involves additional considerations in addition to those shown in Figure 21. First of all, because of per-recipient whitelisting, a message can be directly transferred to one set of recipients and simultaneously rejected or quarantined for the remaining recipients. In the original preferred embodiment, a message that failed Active Filtering was rejected for all recipients. However, because of per-recipient whitelisting, the proxy must provide for splitting the message text from the remote host into two identical parts, with one part being sent to the MTA and the other sent to the quarantine file. The second complication is that there are additional data paths necessary to handle messages that are rejected outright (that is, with no collection of data) and messages that are saved to a quarantine file. At step 1661 the proxy determines if the message was accepted for at least one recipient. All mail from trusted hosts, hosts that successfully pass Active Filtering, and mail authorized by recipient whitelists follows this flow of processing. If the email message is authorized for at least one recipient, then the proxy transfers data between the remote host and the MTA as shown in Figure 21 (steps 1485-1498). That is, the proxy receives the DATA command, message header, message text, and the "." and QUIT commands from the remote host and transfers them transparently (except for Bcc filtering) to the MTA. Where responses are required by the SMTP protocol, the proxy accepts the response from the MTA and forwards it back to the remote host. In addition, the proxy also checks for the existence of an open quarantine file each time it reads a line of text from the remote host. If the quarantine file is open (step 1675), the proxy duplicates the data stream and writes the line to the quarantine file in step 1676. Otherwise, if the quarantine file is not open, then processing is the same as shown in Figure 21. With respect to step 1661, if the current message is not authorized for any recipients, then the proxy has not opened a connection to the MTA. This is the flow that is followed for most junk mail, where Active Filtering rejects the message and the message is not authorized by any recipient whitelists. If the remote host sends a QUIT or a DATA message with no valid recipients, this indicates that the message was identified as a junk mail risk by the Active Filtering tests but that no recipients had a whitelist that matched the MAIL From address. In step 1662, the proxy appends the IP address of the remote host to the blacklist database 1095. The proxy checks for the existence of the autoblacklist flag in the configuration database 1098. If the flag exists, the proxy appends the IP address to the blacklist, otherwise the IP address is not added to the database. In step 1663 the proxy checks for existence of an open quarantine file. If the quarantine file exists, the proxy appends the DATA line (1665) and sends a response (1666) in accordance with the SMTP protocol. Otherwise, if there is no quarantine file, then the proxy sends a 550 error response to the remote host, logs the event, and closes the data connection 1403, as shown in step 1664. If a quarantine file exists, the proxy waits for the remote host to send the message header and text, one line at a time, as shown in Figure 21, steps 1488-1495. At step 1667 the proxy reads each line from the remote host. At step 1668 the proxy compares each line of text with the SMTP end-of-message indicator (a period on a line by itself). If the received line of text is not an end-of-message, the proxy appends the line of text to the quarantine file. Optionally, not shown, the proxy may perform a maximum length check at this point to help deter attempts by attackers to use up all the proxy's disk space. Otherwise, if the current line indicates the end of message, the proxy closes the quarantine file (step 1670), handles the closing SMTP protocol with the remote host, closes the data connection, and exits. If the proxy finds a modebcc entry in the configuration database 1098, it performs Bcc testing as shown in Figure 21, 1488-1492. As the proxy scans each line of the message header, it looks for a local domain name in To:, Cc:, or continuation lines. If the message is not whitelisted or otherwise trusted, and a local domain was not found, the proxy rejects the message and closes the connection. One of the advantages of quarantining rejected messages is that the message is available for review by administrators or by intended recipients, and can be forwarded to the MTA if it is a legitimate message. Various methods can be used to access the quarantine database, including the methods shown in Figure 29 for forwarding desired messages to the MTA. As shown in Figure 29, the proxy server includes the Active Filtering Proxy program 1104 that handles incoming SMTP connections from remote hosts on the Internet 1100. If a message meets the quarantine criteria described earlier, the proxy stores the message in the Quarantine Database 1610 and optionally may append the IP address of the remote host to the Blacklist Database 1095. The system shown in Figure 29 provides two methods for retrieving quarantined messages. An administrator can run the Quarantine Administrator (qadmin) utility 1681 on the proxy server host to forward a quarantined message to all recipients listed in the quarantine file. Users can run a Quarantine Client (qc) program 1684 on a workstation 1683 or MTA 1402 to list and retrieve their quarantined messages. The qadmin program 1681 opens a quarantine file (specified as a calling argument), gets the IP address and port number of the MTA from the Configuration Database 1098, and establishes a TCP connection 1682 to the MTA. It sends the MAIL From, RCPT To:, and DATA commands from the quarantine file to the MTA, then reads the rest of the message from the quarantine file and transfers it line-by-line to the MTA. After successfully transmitting the message, the qadmin program removes any entries in the blacklist database 1095 that match the remote host's IP address, appends the sender's MAIL From address to each recipient's user whitelist 1600, and removes the quarantine file from the quarantine database 1610. A user can run the Quarantine Client (qc) program 1684 on any workstation or server. This program permits the user to list all messages in the quarantine database 1610 that were addressed to the user and to forward selected messages from that list to the MTA 1402. The qc client creates the user address by getting the user's login name from the operating system and appending the local domain name. The Quarantine Protocol 1685 preferably provides cryptographic information (e.g., credentials) in addition to the user's email address or a log in by the user to the qs server in order to prevent spoofing of other users' email addresses. The qc program opens a TCP connection 1685 to the proxy server host 1401 and interacts with a Quarantine Server (qs) program 1686 to access the user's quarantine files. The user can request qc to provide a listing of all quarantined messages addressed to that user. Each line of the report represents a single quarantine message: Name From Subject qf03051459-21481 sender@somewhere.dom Re: computer bid qfO3051522-21552 dude@elsewhere.jp MAKE MONEY FAST!!! ! The file name identifies the date and time of the message (e.g., 03051459 is March 5 at 2:59pm), the MAIL From address, and the Subject line from the message. The qs server gets the quarantine directory from the Configuration Database 1098, reads the directory entries, and opens each file in the directory searching for a RCPT line listing the exact user address sent by the qc client. For each match, it returns the file name, sender, and Subject to the client, which formats the output and caches the information for subsequent listings. Based upon the information in the listing, the user may choose to forward a particular message from the quarantine database 1610 to the MTA 1402. For example, if the user requests the message qf03051459-21481, the qc client 1684 sends the file name to qs 1686, which verifies that the client's email address is listed as a recipient of the message, opens the file, establishes a connection 1682 with the MTA, and transfers the message to the MTA. The MTA then queues the message to the user's mailbox. After successfully transmitting the message, the qs program removes any entries in the blacklist database 1095 that match the remote host's IP address and appends the sender's MAIL From address to the current user's whitelist 1600. If the current user is the final recipient of the message, the qs program removes the quarantine file from the quarantine database 1610. Otherwise, it removes the RCPT entry for the current user and writes the quarantine file back to storage. The system (both the retrieval program and proxy) provides adaptive filter management. Retrieval of a quarantined message automatically configures the blacklist and recipient whitelist to permit subsequent messages from the sender of the quarantined message. In the preferred embodiment, retrieval of a quarantined message causes qadmin or qs to check the blacklist database 1095 and automatically removes any filters matching the IP address. The rationale for this is that if the message is legitimate (non-spam), then the remote host that sent the message should not be blacklisted. This design distributes responsibility to the users of the network for reviewing and forwarding quarantined messages and, thus, removing blacklist entries for legitimate hosts. In addition, retrieval of a quarantined message preferably automatically results in the sender's MAIL From address being appended to the recipient's user whitelist 1600. For example, a remote open relay host can send a sequence of messages to local users. As a message arrives, the proxy blacklists the remote host because it is an open relay. Subsequently, if a user retrieves the message, the retrieval program (qadmin or qs) removes the blacklist entry 1095 for the remote host and appends the sender's MAIL from address to the user's whitelist 1600. Addition of the whitelist entry matching the sender's address automatically prevents further blacklisting, even though the remote host still fails Active Dialup (1420-1423), Active User (1901-1913), or Active Relay (1450-1465) tests. The addition of a whitelist entry matching the sender suffices to prevent further blacklisting. Thus, the entire process of detection, blacklisting, quarantining, blacklist removal and acceptance of the sender adapts to the reactions of local users and is performed without administrative involvement. Other alternative embodiments include web-based or email autoresponder approaches for retrieval of quarantined messages. For example, with a web-based approach, the user could run a web browser that accesses a HTTP server and a database application on the proxy to retrieve listings from the quarantine database. With this approach, the user can list and forward quarantined messages by clicking a search button, or can read messages via the browser interface. In this case, the retrieval server should preferably (a) require the user to log in to the server process and (b) restrict access by a user to only the messages where the user is listed as a recipient of the message. Otherwise, an inquisitive user would be able to browse through all quarantined messages, even those addressed to other users, by simply guessing at file names. This preferred embodiment describes retrieval of messages quarantined by an Active Filtering proxy server. However, the automatic removal of blacklist entries, and the automatic addition of a user whitelist entry following retrieval of a quarantined message is not limited to Active Filtering embodiments. The automatic removal of blacklist entries during retrieval of a quarantined message can be used in any filtering embodiment having a blacklist, quarantine storage, and a quarantine retrieval mechanism. Similarly, the automatic addition of a user whitelist entry during retrieval of a quarantined message can be used in any filtering embodiment having per-recipient whitelists, quarantined storage and a quarantine retrieval mechanism. Though the preferred embodiment uses separate whitelist files, other embodiments can provide the same general capability, for example, use of a single database containing authorized (sender, recipient) pairs, use of wildcards, or use of accept/deny authorizations as are typically used in access control lists. Retrieval of a quarantined message automatically updates the user's respective whitelist 1600. In addition, a user can also edit his or her whitelist by making add, delete, or list requests to the Quarantine Client (qc) program 1684. The qc program sends the edit requests to the Quarantine Server program 1686, which edits the whitelist file for the user. Though the whitelist database 1600 and quarantining have been described for use with the combination of Active Dialup, Active Relay and Active User tests, the whitelisting and quarantining can be used with any single test, or independently. In addition, the whitelisting and quarantining can be used together or separately. Other mechanisms can also be incorporated into the proxy filter, such as a check for domain existence and a content match. The domain existence function checks for the existence of the MAIL From domain. The content match checks for keywords in the MAIL From address. If the message contains any word in this list, the message is rejected. Though the invention has been described for current spamming techniques, such as specific relaying, dialup and user methods, the invention should not be construed as limited to these current approaches. The invention can be implemented to address any developed relaying, dialup and user (forgery) attacks, and can include any suitable per-recipient whitelisting and/or quarantining. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims: 1. A system for selectively accepting an electronic message sent from a sender through a remote host (1400) over a connection to a recipient at a server, the system comprising a dialup filter (1420) determining whether the connection is a dialup connection and accepting the electronic message if the connection is determined to not be a dialup connection. 2. The system of claim 1, wherein the connection is determined to be a dialup connection if the remote host is a dialup. 3. The system of claim 1, further comprising a recipient whitelist database (1600) including a list of acceptable sender addresses for the recipient, wherein said system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600). 4. The system of claim 3, wherein if the sender address is not in said recipient whitelist database (1600), then the dialup filter determines if the connection is a dialup connection. 5. The system of claim 3, wherein if the dialup filter determines that the connection is a dialup connection, then said system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600). 6. The system of claim 3, wherein said system rejects the electronic message for that recipient if said dialup filter determines that the connection is a dialup connection, the sender address is not in said recipient whitelist database (1600) and the dialup filter is not flagged for quarantining. 7. The system of claim 3, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the connection is determined to be a dialup connection and the sender is not matched in any recipient whitelist database (1600). 8. The system of claim 3, further comprising a quarantine database (1610), wherein if said dialup filter determines that the connection is a dialup connection, the sender address is not in said recipient whitelist database (1600) and the dialup filter is flagged for quarantining, then the electronic message is quarantined for that recipient in said quarantine database. 9. The system of claim 8, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the connection is determined to be a dialup connection and the sender is not matched in any recipient whitelist database (1600), and wherein when the quarantined electronic message is retrieved from the quarantine database, the blacklisted network address for that remote host is removed from said blacklist database. 10. The system of claim 8, wherein when the quarantined electronic message is retrieved from the quarantine database, the sender's address is added to the recipient's whitelist database (1600). 11. The system of claim 1, said dialup filter determining whether the connection is a dialup connection based upon a remote host name and a name for at least one host neighboring the remote host. 12. The system of claim 1, wherein said system attempts to establish a reverse connection from said system to the remote host, wherein if the reverse connection cannot be established then said dialup filter determines whether the connection is a dialup connection. 13. The system of claim 12, further comprising a relay filter, wherein if the reverse connection is established then said relay filter determines whether the remote host is an open relay. 14. A system for selectively accepting an electronic message sent from a sender through a remote host (1400) to a recipient at a server, the system comprising a relay filter (1450) determining whether the remote host is an open relay and accepting the electronic message if the remote host is not an open relay. 15. The system of claim 14, further comprising a recipient whitelist database (1600) including a list of acceptable sender addresses for the recipient, wherein said system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600). 16. The system of claim 15, wherein said system rejects the electronic message for that recipient if said relay filter determines that the remote host (1400) is an open relay, the sender address is not in said recipient whitelist database (1600) and the relay filter is not flagged for quarantining. 17. The system of claim 15, further comprising a quarantine database (1610), wherein if said relay filter determines that the remote host is an open relay, the sender address is not in said recipient whitelist database (1600) and the relay filter is flagged for quarantining, then the electronic message is quarantined for that recipient in said quarantine database. 18. The system of claim 17, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the remote host is determined to be an open relay and the sender is not matched in any recipient whitelist database (1600), and wherein when the quarantined electronic message is retrieved from the quarantine database, the blacklisted network address for that remote host is removed from said blacklist database. 19. The system of claim 17, wherein when the quarantined electronic message is retrieved from the quarantine database, the sender's address is added to the recipient's whitelist database (1600). 20. The system of claim 15, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the remote host is determined to be an open relay and the sender is not matched in any recipient whitelist database (1600). 21. The system of claim 14, said relay filter establishing a reverse connection from said system to the remote host and initiating a test transaction to the remote host from an unrelated domain, the test transaction being addressed to a test address at a domain that is unrelated to the remote host, and said relay filter determining that the remote host is an open relay if the remote host accepts the test transaction. 22. The system of claim 14, wherein a test electronic message is addressed to the sender and said relay filter determines that the remote host may be an open relay if the remote host rejects the test electronic message. 23. A system for selectively accepting an electronic message sent from a sender at a sender's domain through a remote host (1400) to a recipient at a server, the system comprising a user filter (1900, 1901) verifying whether the sender of the electronic message is authorized by the sender's domain and accepting the electronic message if the sender of the electronic message is verified as being authorized. 24. The system of claim 23, further comprising a recipient whitelist database (1600) including a list of acceptable sender addresses for the recipient, wherein said system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600). 25. The system of claim 24, wherein said system rejects the electronic message for that recipient if said user filter determines that the sender is not authorized, the sender address is not in said recipient whitelist database (1600) and the dialup filter is not flagged for quarantining. 26. The system of claim 24, further comprising a quarantine database (1610), wherein if said user filter determines that the user is not authorized, the sender address is not in said recipient whitelist database (1600) and the user filter is flagged for quarantining, then the electronic message is quarantined for that recipient in said quarantine database. 27. The system of claim 26, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the sender is determined not to be authorized and the sender is not matched in any recipient whitelist database (1600), and wherein when the quarantined electronic message is retrieved from the quarantine database, the blacklisted network address for that remote host is removed from said blacklist database. 28. The system of claim 26, wherein when the quarantined electronic message is retrieved from the quarantine database, the sender's address is added to the recipient's whitelist database (1600). 29. The system of claim 24, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if the sender is determined not to be authonzed and the sender is not matched in any recipient whitelist database (1600). 30. The system of claim 23, wherein the user filter establishes a test connection to a mailhost that is configured for a sender's domain, and initiates a test transaction to the sender's address, said user filter determining that the user's address is not authorized if the configured mail host does not accept the test transaction. 31. A system for selectively accepting an electronic message sent from a sender through a remote host (1400) to a recipient at a server, the system comprising at least one filter (1420, 1450, 1900, 1901, 1491) that determines whether the electronic message is undesirable, and a quarantine database (1610) for quarantining the electronic message for that recipient if said at least one filter determines that the electronic message is undesirable. 32. The system of claim 31, wherein said quarantine database only quarantines the undesirable electronic message for the recipient if said at least one filter is flagged for quarantining, otherwise the electronic message determined to be undesirable is rejected for the recipient. 33. The system of claim 32, further comprising a recipient whitelist database (1600) including a list of acceptable sender addresses for the recipient, wherein when the quarantined electronic message is retrieved from the quarantine database, the sender's address is added to the recipient's whitelist database (1600). 34. The system of claim 33, further comprising a blacklist database (1095) including a list of blacklisted remote hosts, wherein if the electronic message is determined to be undesirable, and the sender address is not in any recipient whitelist database (1600), then the remote host is added to said blacklist database. 35. The system of claim 32, further comprising a blacklist database (1095) including a list of blacklisted remote hosts, wherein if the quarantined electronic message is retrieved from said quarantine database, then the blacklisted remote host is removed from said blacklist database. 36. The system of claim 31, further comprising a recipient whitelist database (1600) including a list of acceptable sender addresses for the recipient, wherein the electronic message determined to be undesirable is accepted by the system if the sender address for that electronic message is matched in any recipient whitelist database (1600). 37. A system for selectively accepting an electronic message sent from a sender through a remote host (1400) to a recipient at a server, the system comprising at least one filter (1420, 1450, 1900, 1901, 1491) that determines whether an electronic message is undesirable, and a recipient whitelist database (1600) including a list of acceptable sender addresses, wherein the system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600). 38. The system of claim 37, further comprising a quarantine database (1610), wherein if said at least one filter determines that the electronic message is undesirable and the sender address is not in said recipient whitelist database (1600), then the electronic message is quarantined for that recipient in said quarantine database. 39. The system of claim 38, wherein if the quarantined electronic message is retrieved from said quarantine database, then the sender address is added to said recipient whitelist database (1600). 40. The system of claim 38, further comprising a blacklist database (1095) including a list of blacklisted remote hosts, wherein if the quarantined electronic message is retrieved from said quarantine database, then the blacklisted remote host is removed from said blacklist database. 41. The system of claim 37, further comprising a blacklist database (1095) having a list of blacklisted network addresses, wherein the remote host is added to the blacklist database if any of the at least one filter determines that the electronic message is undesirable and the sender address is not matched in any recipient whitelist database (1600). 42. The system of claim 37, wherein if the sender address is not in said recipient whitelist database (1600), then the at least one filter determines if the electronic message is undesirable. 43. The system of claim 37, wherein if the at least one filter determines that the message is undesirable, then said system accepts the electronic message for that recipient if the sender address is in said recipient whitelist database (1600).

Inventors: DONALDSON ALBERT L, DONALDSON, ALBERT, L.
Applicants: DONALDSON ALBERT L, ESCOM CORP, DONALDSON, ALBERT, L., ESCOM CORPORATION
Application year: 2000
Main classification: G06F15
IPC codes: H04L12, H04L29