Simple Mail Transfer Protocol
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Simple Mail Transfer Protocol (SMTP) is the internet standard that handles sending and relaying emails from one computer to another. It matters because it's the foundational technology that makes email delivery possible across the internet.
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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.