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 server in an Internet Protocol (IP) network. POP version 3 (POP3) is the most recent level of development in common use. While many mail services use IMAP today, POP is still a very useful protocol for mail handling.
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 client system, and delete them from the server. Other protocols, notably the Internet Message Access Protocol (IMAP) provide more features of message management to typical mailbox operations. 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.
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.
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 its development 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 19.
RFC 1939 APOP support indicated by <firstname.lastname@example.org> here:
S: <wait for connection on TCP port 110> C: <open connection> S: +OK POP3 server ready <email@example.com> 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
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