Wikipedia
The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today Mail servers and other message transfer agents use SMTP to send and receive mail messages SMTP servers commonly use the Transmission Control Protocol on port number 25 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 as per RFC 8314
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