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 mail server.
POP version 3 (POP3) is the version in common use circa 2019.
POP3 supports download-and-delete from a remote mailbox (“maildrop” in POP3 terminology). Although most POP3 clients have an option to leave mail on server after download, they generally connect, retrieve all messages, store them on the client system, and delete them from the server. (By contrast, the Internet Message Access Protocol (IMAP) normally leaves all messages on the server).
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 POP3 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.