SMTP

networking

Stands for: Simple Mail Transfer Protocol

The protocol that moves email between mail servers.

Simple Mail Transfer Protocol (RFC 5321) handles the sending and relaying of email. Retrieval is done by IMAP or POP3; SMTP is the delivery side.

SMTP moves mail between servers and was designed when the network was small and trusted, which is why the protocol itself has no authentication of the sender. Anyone can claim to be anyone, and everything built since exists to compensate.

That compensation is the modern reality of mail: SPF states which hosts may send for a domain, DKIM signs messages so alteration is detectable, and DMARC ties the two to the visible from address and tells receivers what to do on failure. Deliverability is therefore a configuration problem in DNS more than in the mail server, and a domain that sends no mail should still publish records saying so, because silence is what makes impersonation easy.

Also known as: smtp

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