SMTP:

SMTP, short for Simple Mail Transfer Protocol, is a Transmission Control Protocol/Internet Protocol (TCP/IP) protocol used for sending and receiving e-mail. However, because SMTP is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or Internet Message Access Protocol (IMAP), which enable the user to save messages in a server mailbox and download them periodically from the server. Messaging applications typically use SMTP for sending e-mail and either POP3 or IMAP for downloading messages that have been received for them by the mail server.

SMTP is usually implemented to operate over Transmission Control Protocol (TCP) port 25. You can find the details of SMTP in Request for Comments 821 of the Internet Engineering Task Force (IETF). An alternative to SMTP that is widely used in Europe is X.400.

See also:

Chili!Mail (SMTP Component) in "Chapter 5: Developer’s Reference"

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