How SMTP works
To deliver a message, the sending server looks up the recipient domain's MX record, opens a TCP connection and passes the message through a dialogue of commands (HELO, MAIL FROM, RCPT TO, DATA). Servers talk to each other on port 25, while mail clients submit outgoing mail on port 587 or 465 with encryption.
SMTP and other protocols
SMTP only handles sending, not reading mail:
- Sending — SMTP carries a message from the client to the server and onward between servers.
- Retrieval — messages are pulled from the mailbox by IMAP or POP3 instead of SMTP.
- Authenticity — SPF and DKIM confirm that the sender is genuine.
So SMTP moves mail across the network, while related technologies handle reading it and guarding against forgery.
