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

The Simple Mail Transfer Protocol is implemented to operate over Internet port 25. An alternative to Simple Mail Transfer Protocol is X.400 which is widely used in Europe. Currently, many mail servers support Extended Simple Mail Transfer Protocol (ESMTP), which allows the multimedia files to be delivered as an e-mail.

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