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POP is an Internet mail server protocol that provides an incoming message storage system. It works in conjunction with the SMTP (Simple Mail Transfer Protocol), which provides the message transport services required to move mail from one system to another. The current version is called POP3, as defined in RFC 1939 (Post Office Protocol-Version 3, May 1996).

One goal of POP3 developers was to keep the protocol simple. If more sophisticated operations are needed, IMAP4 (Internet Message Access Protocol) should be considered. IMAP is covered elsewhere.

Several other RFCs are related to POP. RFC 2449 (POP3 Extension Mechanism, November 1998) describes POP3 extensions. RFC 2595 (Using TLS with IMAP, POP3 and ACAP, June 1999) describes security options for POP. RFC 1734 (POP3 AUTHentication command, December 1994) describes the optional AUTH command for indicating an authentication mechanism to the server, performing an authentication protocol exchange, and optionally negotiating a protection mechanism for subsequent protocol interactions.