Abstract:
Disclosed is a system and method for selective email processing. A traffic separator includes an interface for receiving electronic mail traffic from a source network address. The traffic separator also includes a processor for comparing the source network address to a stored list of network addresses to determine a categorization of the network source address. The traffic separator also includes at least one interface for forwarding the electronic mail traffic to one of many message transfer agents (MTAs) based upon said determination. A database stores the list of network addresses. In one embodiment, one or more network addresses in the stored list are network address ranges.

Description:
This application is a continuation of U.S. patent application Ser. No. 11/050,090, filed Feb. 3, 2005, which claims the benefit of U.S. Provisional Application No. 60/541,669, filed Feb. 4, 2004, both of which is are entirely incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic mail, and more particularly to reducing unwanted email by reducing the resources devoted to processing the unwanted email. 
     As the popularity of the Internet has increased drastically over the past few decades, communication via email has often become a large part of people&#39;s daily lives. 
     Unsolicited commercial email, also known as spam, has grown dramatically and has had a significant detrimental impact on computer users and networks. Spam wastes tangible resources relied upon by Internet service providers (ISPs) such as bandwidth, ISP disk space, user email storage space, networking and computer resources, etc. In some instances, spam can bring down servers. 
     One solution to the spam problem is the use of filtering techniques on a per message basis. Spam filters attempt to intercept spam before it reaches an end user&#39;s electronic mailbox. These filters can operate at an ISP or corporate email server (or locally, on an end user&#39;s computer) in order to filter the email before an end user sees the email. Spam filters generally use some form of syntactic or semantic filtering. For example, some filters may have a database of keywords which, if present in an email message, results in the email message being identified as spam. More sophisticated filters use rules that are heuristics used to assign a score to the mail message to be examined, with the score indicating the likelihood of the message being spam. Once a message is identified as spam, it may be deleted, stored in a separate mailbox associated with likely spam messages, or otherwise segregated. 
     While filtering can be effective in decreasing the amount of spam sent to an end user, the reduction in spam is often expensive. The ISP or enterprise mail system has to devote resources to process all incoming messages, including spam. In order to handle the immense and growing volume of email, ISPs and email providers typically have to continually maintain, upgrade, and purchase improved, more powerful and greater numbers of computers and networking resources. 
     A deficiency of current solutions to spam is that email sent from a source has to obtain a connection at the receiving ISP system before the receiving ISP system can identify email as spam. In particular, Message Transfer Agents (MTAs) typically handle the details of sending and receiving email across a network such as the Internet. By convention, the sending MTA (e.g., Unix sendmail or Microsoft Exchange) establishes a connection to the destination MTA. Once the connection is established, email is transferred across the Internet. Thus, existing ISP systems have to receive and process legitimate email as well as spam with a limited amount of resources having a limited number of connections. Because of the enormous volume of email, the limited number of connections available on an ISP&#39;s MTAs and, similarly, the limited amount of resources to handle the enormous volume, often result in a bottleneck to email transferring and processing. Specifically, legitimate email competes with spam for the valuable connections and processing resources, and, as a result, can be delayed. 
     Another solution to the spam problem occurs at the network source level. A “whitelist”, or list of email sources that are known not to deliver large amounts of spam (i.e., “trusted” sources) is created. If email is received at a router from a network source that is not on a whitelist, then email from that source is blocked at the router. The problem is that this can exclude legitimate email that happens not to pass through one of the trusted sources. 
     Thus, spam represents a drain on the efficiency and profitability of ISPs and email providers alike. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides for an improved method and apparatus for processing electronic mail. In accordance with the invention, a traffic separator receives electronic mail traffic from a source network address. The traffic separator compares the source network address to a stored list of network addresses to determine a categorization of the network source address. The traffic separator forwards the electronic mail traffic to one of a plurality of message transfer agents (MTAs) based upon the determination. 
     In accordance with one embodiment of the invention, the categorization of the network source address includes determining a level of trust associated with the network source address. Each MTA (or group of MTAs) is associated with a different level of trust. The database stores a list of network addresses associating sources with different levels of trust. There may be any number of levels of trust. By associating network source addresses and MTAs with a level of trust, email from an untrusted source is directed to a particular MTA. Further, an ISP may provide a greater number of and better resources to the MTAs associated with trusted sources. Thus, the resources available in an ISP&#39;s system and, similarly, the available connections on those resources (i.e., MTAs), are more readily available to receive and process email transmitted from more trusted sources. 
     The traffic separator may be a router. In another embodiment, the traffic separator may be implemented as a load balancer. 
     Further processing of the electronic mail traffic may also be performed after the electronic mail traffic is forwarded to an MTA. This processing may include spam and/or virus filtering. The amount of additional processing performed may vary depending on the level of trust associated with the MTA. The electronic mail traffic can then be forwarded to a message store infrastructure. 
     These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a high level block diagram of an inbound electronic mail architecture in accordance with an embodiment of the invention; 
         FIG. 2  shows a data structure which may be used to store a list of network addresses and associated MTA identifiers in a database; 
         FIG. 3  is a flowchart showing the steps performed by a traffic separator in accordance with an embodiment of the invention; 
         FIG. 4  shows a high level block diagram of a traffic separator which may be used in an embodiment of the invention; 
         FIG. 5  shows an inbound electronic mail architecture in which a router embodiment of the invention may be implemented; 
         FIG. 6  shows an inbound electronic mail architecture in which a load balancer embodiment of the invention may be implemented; 
         FIG. 7A  shows a data structure which may be used to store a list of network addresses and associated router output ports in a database; and 
         FIG. 7B  shows a data structure which may be used to store a list of network addresses and associated Ethernet addresses in a database. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a high level block diagram of an ISP&#39;s inbound electronic mail (i.e., email) architecture in accordance with the principles of the present invention. Further details regarding particular embodiments of the invention will be described in further detail in connection with  FIGS. 2-7B .  FIG. 1  shows a traffic separator  102  receiving email traffic from the Internet  104 . In the past, all email received from the Internet  104  competes for the limited resources and the limited number of connections of the ISP&#39;s resources. In particular, all email (i.e., legitimate email and spam) competes for a connection on the ISP&#39;s MTAs (e.g., MTA  106   a ,  106   b , . . .  106   n  (generally  106 )). Thus, in order for the ISP to identify and prevent spam from being sent to the end user, spam and legitimate email have to obtain a connection on one of the MTAs for processing. Once the connection is obtained, the email is then processed, thereby potentially enabling the MTA&#39;s identification of and filtering of spam. Due to the large volume of email, an ISP may have to devote a large number of MTAs (e.g., thirty to forty MTAs) to receive and process all of the email traffic (i.e., legitimate email and spam) where a large percentage of the total traffic is spam. 
     In accordance with the principles of the present invention, the traffic separator  102  operates at the network or link level to selectively direct incoming email traffic to one of the MTAs  106 . The MTAs  106  are any machine or device that handles the details of sending and receiving email across a network such as the Internet  104 . The MTAs  106  may be individual computers or may be clusters configured for high volumes and/or high availability. In one embodiment, multiple MTAs  106  execute on a single computer. As shown, the email architecture of  FIG. 1  may have any number of MTAs communicating with the traffic separator  102 . The traffic separator  102  determines which MTA  106  to forward a received email message to depending on a list of network addresses stored by database  108 . The database  108  may store one list or multiple lists of network addresses. It is noted that database  108  is shown as an external component connected to traffic separator  102 . However, in various alternative embodiments, the database  108  may be internal to traffic separator  102  (e.g., stored in internal memory or storage), may be an externally connected device such as shown, or may be a stand-alone network node which the traffic separator  102  accesses via a network interface. 
     Each MTA  106  (or group of MTAs) is associated with a different level of trust. The database  108  stores a list of network addresses associating sources with different levels of trust. There may be any number of levels of trust. For example, there may be a “trusted” level, a “somewhat trusted” level, an “unknown” level, a “somewhat untrusted” level, and an “untrusted” level. In particular, trusted sources are sources that are known in advance to not typically transmit spam. If a network source address does not appear in the list of network addresses in the database  108 , the level of trust is “unknown”. A network source address is classified as untrusted if spam is typically received from the network source address. 
     For example, the first MTA  106  may be configured to meet committed performance levels of the ISP and, therefore, may be associated with the highest level of trust. Thus, if the traffic separator  102  determines that email traffic has a source network address that is trusted, the traffic separator  102  directs the email traffic to the first MTA  106   a . In one embodiment the second MTA  106   b  may be engineered to lower performance levels relative to the first MTA  106   a . Thus, if the traffic separator  102  determines that email traffic has a source network address that is somewhat trusted, the traffic separator  102  directs the email traffic to the second MTA  106   b . Further, an ISP may devote lower amounts of and less powerful resources to receive and process spam. To prevent spam from monopolizing connections on multiple MTAs  106 , a single MTA (e.g., MTA  106   n ) may be designated as the MTA for email traffic received from an untrusted network source. By associating one or more levels of trust to each MTA  106  (or group of MTAs) and email sources, much less competition arises between spam and legitimate email for valuable MTA connections. Specifically, because the traffic separator  102  selectively directs email traffic to particular MTAs  106  based on the level of trust associated with the source of the email traffic, the email traffic is not all competing for the same MTA connections. 
     Although particular trust levels are described with respect to particular MTAs  106  (e.g., the trusted email traffic is sent to the first MTA  106   a ), the traffic separator  102  can direct the email traffic associated with a particular trust level to a group of MTAs  106 . 
     In further embodiments, one or more MTAs  106  are designated as spare MTAs. The spare MTA can be employed if an active MTA  106  fails. In one embodiment, the traffic separator  102  detects a failure of an MTA  106  and automatically transmits packets to the spare MTA in place of the failed MTA  106 . 
     After the MTA  106  receives the email traffic, additional processing on the email may be performed. This additional processing may be performed by a corresponding spam/virus filtering function  110   a ,  110   b ,  110   n  (generally  110 ). The spam/virus filtering function  110  can be implemented in a number of ways, such as with a function call by the MTA  106  or via a software program executing on an independent machine or device. In accordance with an advantage of the invention, the spam/virus filtering function  110  may perform a different amount of processing (e.g., filtering) for messages arriving from the various MTAs  106 . Thus, because the traffic separator  102  only transmits email traffic from trusted sources to the first MTA  106   a , the amount of additional processing (e.g., filtering) performed on these emails may be minimal. Thus, the corresponding first spam/virus filtering function  110   a  may perform minimal filtering. In some embodiments, the email traffic from the first MTA  106  is transmitted directly to the message store infrastructure  112  (e.g., without further processing). The message store infrastructure  112  may be, for example, one or more email servers. Further, the corresponding second spam/virus filtering function  110  may perform more aggressive filtering for email traffic received from the second MTA  106   b  because the email traffic is from a source that is somewhat trusted rather than trusted. Once this additional processing is completed, the email traffic is sent to the message store infrastructure  112 . 
       FIG. 2  shows one embodiment of a data structure which may be stored in database  108 . In accordance with one embodiment of the invention, database  108  contains a relational database  200  containing multiple records, with each record comprising multiple fields. Field  202  identifies the source IP address. This identification may be an IP address, a range of IP addresses, or subnetwork addresses that cover a number of individual addresses. Field  204  identifies a level of trust associated with the source IP address. The level of trust may be designated with a flag (e.g., trusted, somewhat trusted, unknown, somewhat untrusted, untrusted) or may be designated with a number (e.g., a 1 corresponds to the trusted level, a 2 corresponds to the somewhat trusted level, etc.). Field  206  identifies the MTA  106  associated with the level of trust. Thus, field  206  identifies which MTA  106  the traffic separator  102  directs email traffic to depending on the level of trust associated with the source IP address of the email traffic. 
     Records  208 - 214  show exemplary records which may be stored in database  108 . Record  208  indicates that the source IP address is 192.200.3.5 and this source network address consistently does not deliver spam. Thus, this source network address is assigned a level of trust of 1. The traffic separator  102  directs email traffic from the source IP address of 192.200.3.5 to the first MTA  106   a . Record  210  indicates a range of IP addresses that fall into a second level of trust. Thus, any IP address that begins with 205 will be routed to the second MTA  106   b . Similarly, if the traffic separator  102  receives email traffic from a source IP address of 63.128.200.18, the traffic separator  102  determines that this network source has a level 5 trust rating (i.e., untrusted) and transmits the email traffic to the last MTA  106   n . Finally, as shown in record  214 , if the traffic separator  102  receives email traffic from an unknown source IP address, the traffic separator  102  determines that this network source has a level 3 trust level (i.e., unknown) and transmits the email traffic to a third MTA  106   c  (not shown). 
     The database records may be populated in various ways. In one embodiment, the database records are populated manually. For example, an administrator can manually update the database  108  by listing trusted sources (as determined from past email traffic). In some embodiments, the lists are text files that are updated via a text editor. Alternatively, a user may update the lists using a graphical user interface (GUI). The lists may be relatively static, rarely needing updating or may be dynamic, requiring updating often (e.g., in near real time). In some embodiments, the lists are updated automatically (e.g., via the Mail Abuse Prevention System (MAPS) Realtime Blackhole List (RBL)). The RBL is a list that is frequently updated with IP addresses of spam sources. 
     In yet another embodiment, the lists are updated adaptively. In this embodiment, the MTAs  106  use heuristics to determine a network source&#39;s classification (e.g., unknown, trusted, etc.). The heuristics may require email messages delivered (from a particular network source) over the same MTA connection to have less than a threshold percentage of unknown recipients before classifying the email source as a trusted source. For example, if more than 10% of the recipients of email messages received from a particular source network address are unknown, then the source of the email message may be classified as “somewhat untrusted”. This information may also be fed back from the MTA  106  to the database  108 . An example of the feedback from MTA  106   n  is shown with feedback arrow  109 . The heuristics may also warrant a classification change back to the unknown level if the heuristics determine that the same email source is sending email traffic having less than 10% of its recipients as unknown recipients. Further, if the traffic separator  102  repeatedly receives email traffic having no unknown recipients from the same IP source, the MTA  106  may then update the database  108  to classify this source as a somewhat trusted source. Other heuristics rules may be applied. 
     An embodiment of the steps performed by the traffic separator  102  of  FIG. 1  will now be described in further detail in connection with the flowchart of  FIG. 3 . The traffic separator  102  receives email traffic from the Internet  104 , as shown in step  302 . The traffic separator  102  determines the network source address of the email traffic. In one embodiment, the traffic separator  102  makes this determination by inspecting the IP header of the email packet. The traffic separator  102  then compares the source network address of the email traffic with a stored list of source network addresses in step  304  to associate packets with a level of trust. The traffic separator  102  then forwards the email traffic to the corresponding MTA  106  based on the database lookup, as shown in step  306 . If the traffic separator  102  determines that the source network address of the email traffic is not on any list in step  304 , the traffic separator  102  considers the source network address as unknown and transmits the email traffic to a MTA  106  designated to receive and process email traffic from an unknown source. 
     Thus, the traffic separator  102  selectively directs email traffic to particular MTAs  106  depending on a comparison of the network source address of the email traffic and a list of stored network source addresses. 
     A high level block diagram of a computer implementation of the traffic separator  402  is shown in  FIG. 4 . Traffic separator  402  contains a processor  404  which controls the overall operation of the computer by executing computer program instructions which define such operation. The computer program instructions may be stored in a storage device  412  (e.g., magnetic disk, database  108 ) and loaded into memory  410  when, execution of the computer program instructions is desired. Thus, the traffic separator operation will be defined by computer program instructions stored in memory  410  and/or storage  412  and the computer will be controlled by processor  404  executing the computer program instructions. Computer  402  also includes one or more input network interfaces  406  for communicating with other devices via a network (e.g., the Internet) and for receiving the email traffic  414 . Computer  402  also includes one or more output network interfaces  416  for communicating with other devices and for transmitting email traffic  418  to other devices. Traffic separator  402  also includes input/output  408  which represents devices which allow for user interaction with the computer  402  (e.g., display, keyboard, mouse, speakers, buttons, etc.). One skilled in the art will recognize that an implementation of an actual computer will contain other components as well, and that  FIG. 4  is a high level representation of some of the components of such a computer for illustrative purposes. 
       FIG. 5  shows the email architecture in which a router embodiment of the invention may be implemented. In one embodiment, the traffic separator is a router  502 . The router  502  receives packets from the Internet  504  directed to the IP network address(es) of the MTAs (e.g., MTA  506   a ,  506   b ,  506   c  (generally MTA  506 )). In one embodiment, the router  502  employs source address based routing. The router  502  has ports  508   a ,  508   b ,  508   c  (generally  508 ) corresponding with the respective MTA  506   a ,  506   b ,  506   c . The router  502  communicates with database  509  to determine which port  508  to use to transmit the email traffic. The router  502  (i.e., each port  508  of the router  502 ) connects to distinct local area network (LAN) segments  510   a ,  510   b ,  510   c  (generally  510 ). Each LAN segment  510  enables communication between the router  502  and a corresponding MTA  506 . Thus, to communicate with the first MTA  506   a , the router  502  communicates over the first LAN segment  510   a . Each MTA  506   a ,  506   b ,  506   c  communicates with a corresponding spam/virus filtering function  512   a ,  512   b ,  512   c  (generally  512 ). Although shown with three MTAs  506 , a router with three ports  508 , three LAN segments  510 , and three spam/virus filtering functions  512 , any number of each may be present according to the principles of the invention. 
     In one embodiment, the traffic separator  502  may be a computer executing the Linux operating system. The Linux kernel provides the routing function. The list of IP addresses is stored as a list of kernel routing rules. 
       FIG. 6  shows the email architecture in which a load balancer embodiment of the invention may be implemented. The load balancer  602  has the IP address of the ISP&#39;s incoming MTA system. The load balancer  602  receives email traffic from the Internet  604 . The load balancer  602  and the MTAs  606   a ,  606   b ,  606   c  (generally  606 ) reside on the same Ethernet LAN segment  608 . The load balancer  602  and the MTAs all have the MTA IP address. 
     Generally, IP networks maintain a mapping between the IP address of a device and its Media Access Control (MAC) address. This mapping is referred to as the Address Resolution Protocol (ARP) table. In accordance with the principles of the present invention, the load balancer  602  responds to the ARP requests associated with the IP address of the MTAs  606 . The MTAs  606  do not respond to the ARP requests. Thus, the load balancer  602  receives all packets destined for the MTA IP address. When an incoming packet arrives, the load balancer  602  performs a database lookup of the stored lists of network source addresses in database  607 . Each list is associated with an MTA  606  (or group of MTAs  606 ). If a packet&#39;s source address is found on one of the stored lists, the load balancer  602  transmits the packet on the Ethernet LAN  608  using the MAC address of the corresponding MTA  606 . If the load balancer  602  determines that the packet is not on a stored list, the load balancer  602  transmits the packet over the Ethernet LAN  608  using the MAC address of the MTA  606  (e.g., MTA  606   c ) designated to handle email traffic from sources not on a list. 
     In further embodiments, one or more MTAs  606  (e.g., a fourth MTA  606   d ) are designated as spare MTAs. The spare MTA  606   d  can be employed if an active MTA  606  fails. In one embodiment, the load balancer  602  detects a failure of an MTA  606  and automatically transmits packets to the spare MTA  606   d  in place of the failed MTA  606 . 
     In one embodiment, the architecture is implemented on a computer executing the Linux operating system. The load balancer  602  uses the “firewall mark” feature of the Linux kernel&#39;s IP packet filtering subsystem to mark packets based on their source IP addresses. The kernel&#39;s IP virtual server subsystem is used to transmit packets to the appropriate MTA system by rewriting the destination MAC address based on the packet marking. 
       FIG. 7A  shows one embodiment of a data structure which may be stored in database  509 . The data structure shown in  FIG. 7A  relates to the router embodiment described above in  FIG. 5 . In accordance with the embodiment of  FIG. 7A , database  509  contains a relational database  700  containing multiple records, with each record comprising multiple fields. Field  702  identifies the source IP address. Field  703  identifies a level of trust associated with the source IP address. Field  704  identifies the router output port that the router  502  directs email traffic to depending on the level of trust associated with the source IP address. 
     Records  706 - 711  show exemplary records which may be stored in database  509 . Record  706  indicates that the source IP address is 192.200.3.5 and this source network address consistently does not deliver spam. Thus, this source network address is assigned a level of trust of 1. The router  502  directs email traffic from the source IP address of 192.200.3.5 to router output port 1 and, consequently, to the first MTA  506   a.    
     Record  708  indicates a range of IP addresses that fall into a second level of trust. Thus, any IP address that begins with 197 will be routed to the output port 2 of the router  502 . Output port 2 communicates with the second MTA  506   b . Similarly, if the router  502  receives email traffic from a source IP address of 63.128.200.18, the router  502  determines that this network source has a level 3 trust rating and transmits the email traffic to the third MTA  506   c  (as shown in record  710 ). Furthermore, if the router  502  receives email traffic from any other source IP address, the router  502  determines that the network source address is not on a list and is therefore unknown. The router  502  transmits the unknown email traffic to, for instance, a router output port 4 and, consequently, to a fourth MTA (shown in record  711 ). 
       FIG. 7B  shows one embodiment of a data structure which may be stored in database  607 . The data structure shown in  FIG. 7B  relates to the load balancer embodiment described above in  FIG. 6 . In accordance with the embodiment of  FIG. 7B , database  607  contains a relational database  712  containing multiple records, with each record comprising multiple fields. As described above, field  714  identifies the source IP address. Field  716  identifies the level of trust associated with the source IP address. Field  718  identifies the list of Ethernet MAC addresses that the load balancer  602  directs email traffic to depending on the level of trust associated with the source IP address. 
     Record  720  indicates a range of IP addresses that fall into the first level of trust. Thus, any emails having a source IP address that begins with 205 will be transmitted to the MTAs having the listed MAC addresses. Similarly, if the load balancer  602  receives email traffic having an IP address of 143.89.1.1, the load balancer  602  transmits the email traffic to the MTA having a MAC Address of 04508712C1B8 (as shown in record  722 ). Further, if the load balancer  602  receives email traffic from any other source IP address, the load balancer  602  determines that the network source address is not on a list and is therefore unknown. The load balancer  602  transmits the unknown email traffic to, for instance, an MTA having a MAC address of 45012732814 (shown in record  724 ). 
     In one embodiment, the MTA associated with untrusted sources has a small number of TCP connections available for email. Thus, if a large volume of email traffic is waiting for a limited number of connections, one or more sending email systems may time out because the sending email systems have not received a connection before a predetermined amount of time has elapsed. This timeout may discourage spammers from sending spam to the ISP. Moreover, the email traffic from the untrusted source is only occupying one MTA rather than tying down multiple MTAs. 
     The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.