The Delivery Protocol
SMTP (Simple Mail Transfer Protocol) is the language servers speak to move mail across the internet. It is a push protocol: a sending server hands a message to a receiving server, which may relay it again until it reaches the recipient's mailbox.
The Conversation
An SMTP exchange is a short text dialog over a TCP connection.
- The client greets with HELO or EHLO and announces itself.
- It states the sender with MAIL FROM and each recipient with RCPT TO.
- It sends the body after the DATA command and ends with a single dot.
- The server replies with numeric codes such as 250 for success.
Submission vs Relay
Your mail client submits outgoing mail on port 587 with authentication. Servers relay between each other on port 25. Final pickup into a client uses IMAP or POP3, not SMTP, because SMTP only moves mail toward a mailbox.
Key idea
SMTP is a push protocol that relays mail server to server using a simple text dialog, while clients fetch delivered mail with IMAP or POP3.