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In computing, the Post Office Protocol (POP) is an "application-layer "Internet standard "protocol used by local "e-mail clients to retrieve "e-mail from a remote "server over a "TCP/IP connection. POP has been developed through several versions, with version 3 (POP3) being the last standard in common use before largely being made obsolete by the more advanced "IMAP as well as "webmail.
POP supports download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP "RFC's). Although most POP clients have an option to leave mail on server after download, e-mail clients using POP generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect. Other protocols, notably IMAP, ("Internet Message Access Protocol) provide more complete and complex remote access to typical "mailbox operations. In the late 1990s and early 2000s, fewer "Internet Service Providers (ISPs) supported IMAP due to the storage space that was required on the ISP's hardware. Contemporary e-mail clients supported POP, then over time popular mail client software added IMAP support.
A POP3 server listens on "well-known port 110. "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 995.
Available messages to the client are fixed when a POP 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 message-number. When the client exits the "session, the mail marked for deletion is removed from the maildrop.
POP1 was specified in RFC 918 (1984), POP2 by RFC 937 (1985).
POP3 originated with RFC 1081 (1988). Its current specification is RFC 1939, updated with an extension mechanism, RFC 2449 and an authentication mechanism in RFC 1734.
The original POP3 specification supported only an unencrypted USER/"PASS "login mechanism or Berkeley ".rhosts access control. POP3 currently 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" exists only as an informal proposal adding basic folder management, multipart message support, as well as message flag management to compete with IMAP; but has not progressed since 2003.
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. Except for 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.
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).
"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.
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 with the differences 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.
The APOP usage is a direct example from RFC 1939 page 18.
RFC 1939 APOP support indicated by <email@example.com> here:
S: <wait for connection on TCP port 110> C: <open connection> S: +OK POP3 server ready <firstname.lastname@example.org> C: APOP mrose c4c9334bac560ecc979e58001b3e22fb S: +OK mrose's maildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +OK dewey POP3 server signing off (maildrop empty) C: <close connection> S: <wait for next connection>
POP3 servers without the optional APOP command expect the client to log in with the USER and PASS commands:
C: USER mrose S: +OK User accepted C: PASS tanstaaf S: +OK Pass accepted