SMTP

Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks.

While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying.  For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes/Domino) to access their mail box accounts on a mail server.

SMTP uses TCP port 25.  The protocol for new submissions (MSA) is effectively the same as SMTP, but it uses port 587 instead.  SMTP connections secured by SSL are known by the shorthand SMTPS, though SMTPS is not a protocol in its own right.