Simple Mail Transport Protocol (SMTP)

Simple Mail Transport Protocol (SMTP) is an IETF standard for the sending of email. SMTP uses the well-known TCP port 25. Email retrieval is performed by other protocols such as POP3 or IMAP4.

See: RFC 2821.

References:

http://www.emailaddressmanager.com/tips/protocol.html

http://www.businesscreatorpro.com/articles/what-is-imap-and-pop3.php