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Definition: SMTP stands for Simple Mail Transfer Protocol. SMTP is a standard network protocol for transmitting messages to an email server on the Internet.

All modern email client programs support SMTP. Web-based clients embed the address of an SMTP server inside their configuration, while PC clients provide SMTP settings that allow users to specify their own server of choice. Because SMTP handles outgoing messages and not incoming ones, email clients require addresses of both an SMTP server and another server that processes inbound messages (usually, POP or IMAP).

SMTP uses TCP port number 25 for standard communication. To improve SMTP and help combat spam on the Internet, standards groups have also designed TCP port 587 to support certain aspects of the protocol. A few Web email services, such as GMail, use the unofficial TCP port 465 for SMTP.

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