Email & SMTP

Email is the world's largest text protocol. Sender writes a message; it traverses the internet through a chain of mail servers; recipient retrieves it via IMAP or POP3.

The protocol that moves email between servers is SMTP (Simple Mail Transfer Protocol, RFC 5321). It dates from 1982 (RFC 821) and has been incrementally extended ever since.

[Sender] -> Submission MTA -> Relay MTA -> Recipient MTA -> [Mailbox]
   ↑           (587)          (25)             (25)
   |           SMTP           SMTP             SMTP
   |
   submit via SMTP/HTTPS API/MAPI

Three roles for SMTP servers:

• MSA (Mail Submission Agent, port 587 or 465): accepts mail from authenticated users.
• MTA (Mail Transfer Agent, port 25): relays mail between servers.
• MDA (Mail Delivery Agent): writes mail to mailbox.

Real implementations: Postfix, Exim, Sendmail (legacy), OpenSMTPD, Microsoft Exchange.

Modern reality:

• Most email today goes through ~5 large providers (Google, Microsoft, Yahoo, Apple, Yandex).
• Spam reputation matters enormously. Self-hosted SMTP routinely blocked.
• Layered protocols on top: SPF, DKIM, DMARC for authentication.

The core SMTP conversation is text. You can telnet smtp.example.com 25 and type commands.