SMTP (Simple Mail Transfer Protocol) is the Internet standard protocol for transmitting and relaying email across the Internet. Defined in RFC 5321, SMTP handles the transmission envelope and message relay, not mailbox retrieval — that is handled by IMAP or POP3.
How it works
SMTP operates over TCP, typically on port 25 for server-to-server relay between mail transfer agents (MTAs). A client opens a connection, sends EHLO to identify itself, then issues envelope commands: MAIL FROM:<sender>, RCPT TO:<recipient> (one or more recipients), and DATA (the message headers and body). The server responds with three-digit status codes (250 = OK, 550 = mailbox unavailable, 421 = service unavailable). The message is relayed hop-by-hop across MTAs until it reaches the recipient's mail server.
For client-to-server submission (authenticated sending by users), RFC 6409 specifies port 587 with mandatory authentication. Port 465 is used for implicit TLS submission. Both support STARTTLS (RFC 3207) to upgrade plaintext connections to encrypted TLS mid-session.
Example
A user sends an email from [email protected] to [email protected]: the mail client connects to smtp.example.com:587, authenticates, sends MAIL FROM:<[email protected]>, RCPT TO:<[email protected]>, and the message data, then the provider's outbound MTA relays it over port 25 (with TLS) to example.org's MX server for delivery.
Security and deliverability
SMTP is often paired with SPF, DKIM, and DMARC to prevent spoofing and improve email deliverability.
WarningSMTP is push-only and cannot download or sync mail — receiving email requires separate IMAP or POP3 configuration. Many ISPs block outbound port 25 to prevent spam, so authenticated submission on port 587 or 465 is typically required.