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SMTP (Simple Mail Transfer Protocol)

Contributor(s): Joseph Mathew
This definition is part of our Essential Guide: Secure email servers on Exchange, Office 365 or both

SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail. On Unix-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server. Microsoft Exchange includes an SMTP server and can also be set up to include POP3 support.

SMTP usually is implemented to operate over Internet port 25. An alternative to SMTP that is widely used in Europe is X.400. Many mail servers now support Extended Simple Mail Transfer Protocol (ESMTP), which allows multimedia files to be delivered as e-mail. 

 

This was last updated in September 2005

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Network protocol for transmitting messages to an email server on the Internet combat spam on the Internet, standards groups have also designed TCP port 587 to support certain aspects of the protocol;
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SMTP is the best.
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Bloody hell Margaret. Since you put incorrect information on you're little article about SMTP sending and receiving emails. I've now failed my Computer Science course. Thanks Margaret!
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Margaret. I am very disappointed with you because this poor boy failed his computer science course and it is all your fault. Thanks Margaret!
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