POP3

Short for Post Office Protocol version 3. It is used to retrieve emails from
remote servers to a local client over a TCP/IP connection.

The earlier versions of the POP protocol, POP (informally called POP1) and
POP2, have been thoroughly obsoleted by POP3. In contemporary usage, the
less precise term "POP" almost always means "POP3" in the context of email
protocols.

POP3 and its predecessors are designed to allow users with intermittent
connections such as dial-up connections to download email when connected,
while the SMTP protocol was designed to allow systems with permanent
connections to receive email. POP3 is a pull protocol (where the client
requests data from the server), whereas SMTP is a push protocol where the
client sends data to the server without the server requesting the data in
advance.

POP3 also supports several authentication methods to provide varying levels
of protection against illegitimate access to a user's email. It is also
possible to encrypt POP3 traffic using SSL.

POP3 works over a TCP/IP connection using port 110.

Other email protocols include SMTP and IMAP.

POP3 RFCs

   * RFC 1939 "Post Office Protocol - Version 3"
   * RFC 2449 "POP3 Extension Mechanism"
   * RFC 1734 "POP3 AUTHentication command"
   * RFC 2222 "Simple Authentication and Security Layer (SASL)"
   * RFC 3206 "The SYS and AUTH POP Response Codes"