The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 465 or 587 per RFC 8314. For retrieving messages, IMAP (which replaced the older POP3) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.

SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. SMTP servers commonly use the Transmission Control Protocol on port number 25 (between servers) and 587 (for submission from authenticated clients), both with or without encryption, and 465 with encryption for submission.

Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers. As more computers were interconnected, especially in the U.S. Government's ARPANET, standards were developed to permit exchange of messages between different operating systems.

Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in RFC 196; and the SNDMSG program, which Ray Tomlinson of BBN adapted that year to send messages across two computers on the ARPANET. A further proposal for a Mail Protocol was made in RFC 524 in June 1973, which was not implemented.

The use of the File Transfer Protocol (FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed.

SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the International Network Working Group in INWG Protocol note 2, written in September 1974. INWG discussed protocols for electronic mail in 1979, which was referenced by Jon Postel in his early work on Internet email. Postel first proposed an Internet Message Protocol in 1979 as part of the Internet Experiment Note (IEN) series.

In 1980, Postel and Suzanne Sluizer published RFC 772 which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. RFC 780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP, an allocation that has since been removed by IANA. In November 1981, Postel published RFC 788 "Simple Mail Transfer Protocol".

The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities.