What Is an SMTP Port Number?


Quick Answer

An SMTP port number is an Internet port through which the protocol operates. A common port number that is used by SMTP is port number 25. An alternative port that is used in Europe is port X.400.

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Full Answer

SMTP (Simple Mail Transfer Protocol) is used when an e-mail is sent or received. As part of TCP/IP (Transmission Control Protocol/Internet Protocol), SMTP works in conjunction with POP3 (Post Office Protocol 3) or IMAP (Internet Message Access Protocol) to allow users to save messages on a mailbox server. In function, the SMTP protocol has difficulty queuing received messages. Some programs utilize it only for sending email. Email systems based on UNIX use an SMTP server called "sendmail."

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