Simple Mail Transfer Protocol
Internet protocol used for relaying e-mails
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 and POP are widely used, 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.
History
Predecessors to SMTP
Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers.
“Simple Mail Transfer Protocol” enters the record as internet protocol used for relaying e-mails. Crown Archives preserves that source wording while asking what Simple, Mail and Transfer can confirm, complicate or overturn.
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“Simple Mail Transfer Protocol” is worth following because a concise public description often conceals a longer documentary argument. Here, Simple, Mail and Transfer provides the most credible route into that argument.
Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Aug 9, 2026. The linked authority identifier is Q160453. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank. The first chronological checks are 1980 and 1971.
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This entry incorporates text from “Simple Mail Transfer Protocol” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.