The Post Office Protocol (POP) is a mailbox access protocol defined by IETF [RFC 1939]. It is used to retrieve e-mail from a mail server. POP is one of the most commonly used mailbox access protocols.
POP3 is a simple protocol that typically downloads all mail for a user from the server, then deletes the copy on the server, although there is an option to maintain it on the server. POP3 is similar SMTP, in that the client connects to a port (normally port 110 or port 995 when using TLS) and sends ASCII commands, to which the server responds. When the session is complete, the client terminates the connection. POP3 transactions are normally done in the clear, but an extension is available to do POP3 over TLS using the STLS command, which is very similar to the STARTTLS option in SMTP. Clients may connect initially over port 110 and invoke the STLS command, or alternatively, most servers allow TLS by default connections on port 995.
- "Overview" section: NIST Special Publication 800-177, at 8.