SMTP (Simple Mail Transfer Protocol)
SMTP (RFC 5321) is the protocol mail servers use to relay email from a sender's outgoing server to a recipient's incoming server, and from a mail client to its own outgoing server before that. It's a store-and-forward protocol: a message can hop through several SMTP servers before reaching its final destination, each one accepting responsibility for onward delivery or returning a bounce if it can't deliver. SMTP on its own has no built-in encryption or sender authentication -- both were added later as extensions, STARTTLS (RFC 3207) for encrypting the connection, and SPF, DKIM and DMARC as separate DNS-based mechanisms for verifying who actually sent a message -- which is why a server running plain SMTP alone cannot guarantee either privacy in transit or that a message's claimed sender is genuine.
By AdminKit team, Netbase JSC · Updated