Post Office Protocol
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common use, and along with IMAP the most common protocols for email retrieval. Purpose. The Post Office Protocol provides access via an Internet Protocol (IP) network for a user client application to a mailbox ("maildrop") maintained on a mail server. The protocol supports download and delete operations for messages. POP3 clients connect, retrieve all messages, store them on the client computer, and  finally delete them from the server. This design of POP and its procedures was driven by the need of users having only temporary Internet connections, such as dial-up access, allowing these users to retrieve e-mail when connected, and subsequently to view and manipulate the retrieved messages when offline. POP3 clients also have an option to leave mail on the server after download. By contrast, the Internet Message Access Protocol (IMAP) was designed to normally leave all messages on the server to permit management with multiple client applications, and to support both connected ("online") and disconnected ("offline") modes of  operation. A POP3 server listens on well-known port number 110 for service requests. Encrypted communication for POP3 is either requested after protocol initiation, using the STLS command, if supported, or by POP3S, which connects to the server using Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-known TCP port number 995. Messages available to the client are determined when a POP3 session opens the maildrop, and are identified by message-number local to that session or, optionally, by a unique identifier assigned to the message by the POP server. This unique identifier is permanent and unique  to the maildrop and allows a client to access the same message in different POP sessions. Mail is retrieved and marked for deletion by the message-number. When the client exits the session, mail marked for deletion is removed from the maildrop. History. The first version of the Post Office Protocol, POP1, was specified in RFC 918 (1984). POP2 was specified in RFC 937 (1985). POP3 is the version in most common use. It originated with RFC 1081 (1988) but the most recent specification is RFC 1939, updated with an extension mechanism (RFC 2449) and an authentication mechanism in RFC  1734. This led to a number of POP implementations such as Pine, POPmail, and other early mail clients. While the original POP3 specification supported only an unencrypted USER/PASS login mechanism or Berkeley .rhosts access control, today POP3 supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail. Most are provided by the POP3 extension mechanisms. POP3 clients support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version. RFC 1460 introduced APOP into the core protocol. APOP is a challenge–response protocol which uses the  MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera Mail, Eudora, KMail, Novell Evolution, RimArts' Becky!, Windows Live Mail, PowerMail, Apple Mail, and Mutt. RFC 1460 was obsoleted by RFC 1725, which was in turn obsoleted by RFC 1939. POP4. POP4 exists only as an informal proposal adding basic folder management, multipart message support, as well as message flag management to compete with IMAP; however, its development has not progressed since 2003. Extensions and specifications. An extension mechanism was proposed in RFC 2449 to accommodate general  extensions as well as announce in an organized manner support for optional commands, such as TOP and UIDL. The RFC did not intend to encourage extensions, and reaffirmed that the role of POP3 is to provide simple support for mainly download-and-delete requirements of mailbox handling. The extensions are termed capabilities and are listed by the CAPA command. With the exception of APOP, the optional commands were included in the initial set of capabilities. Following the lead of ESMTP (RFC 5321), capabilities beginning with an X signify local capabilities. STARTTLS. The STARTTLS extension allows the use of Transport Layer  Security (TLS) or Secure Sockets Layer (SSL) to be negotiated using the "STLS" command, on the standard POP3 port, rather than an alternate. Some clients and servers instead use the alternate-port method, which uses TCP port 995 (POP3S). SDPS. Demon Internet introduced extensions to POP3 that allow multiple accounts per domain, and has become known as "Standard Dial-up POP3 Service" (SDPS). To access each account, the username includes the hostname, as "john@hostname" or "john+hostname". Google Apps uses the same method. Kerberized Post Office Protocol. In computing, local e-mail clients can use the Kerberized Post Office Protocol (KPOP ), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. The KPOP protocol is based on the POP3 protocol – differing in that it adds Kerberos security and that it runs by default over TCP port number 1109 instead of 110. One mail server software implementation is found in the Cyrus IMAP server. Session example. The following POP3 session dialog is an example in RFC 1939: POP3 servers without the optional APOP command expect the client to log in with the USER and PASS commands: Comparison with IMAP. The Internet Message Access  Protocol (IMAP) is an alternative and more recent mailbox access protocol. The highlights of differences are: