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The Simple Mail Transfer Protocol (SMTP) is a protocol of the Internet protocol family that is used to exchange e-mails in computer networks. It is primarily used to feed in and forward e-mails. Other specialized protocols such as POP3 or IMAP are used to retrieve messages. SMTP servers traditionally accept connections on port 25 ("smtp").

Newer servers also use port 587 to receive mail only from authenticated users ("Mail Submission Agent"). By a clear separation of own and foreign users configuration problems and spam should be avoided (→ SMTP-Relay-Server). In addition, due to the different ports, a simple firewall rule can be used to block uncontrolled outgoing spam messages from your own network without completely excluding connections to external SMTP servers.