SMTP (S M T P) stands for Simple Mail Transfer Protocol. It is a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP.

SMTP is commonly used to send messages from a mail client to a mail server (i.e. sending from a mail client application like Microsoft Outlook™.) The new protocol for mail submission (MSA) is essentially the same as SMTP, but it uses port 587 instead. (This new standard is slowly being adopted.)

SMTP was first defined by RFC 821 (1982, eventually declared STD 10),[1] and last updated by RFC 5321 (2008)[2] which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP uses TCP port 25.