Term:SMTP [Simple Mail Transfer Protocol]« Back to Glossary Index
SMTP is an acronym for Simple Mail Transfer Protocol, which is the protocol used to transport email messages from computer to computer until they reach their destination.
SMTP is used between email programs, which are SMTP clients, and SMTP mail servers, which forward email to its destination.
SMTP is then also used between SMTP mail servers as they forward the message on to the next server, until the message reaches its final destination mail server.
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 587 or 465 per RFC 8314. For retrieving messages, IMAP and POP3 are standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.
Since SMTP's introduction in 1981, 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 (for plaintext) and 587 (for encrypted communications).