Implementing SMTP straight from the RFCs

Email feels ancient, and that is exactly why it is a great learning target: the specifications are precise, stable, and freely available. I built an SMTP server and client in Go by reading the RFCs directly.

The command loop

SMTP is a line-based, request/response protocol. The core of the server is a loop that parses commands like HELO, MAIL FROM, RCPT TO, and DATA, and replies with numeric status codes.

C: EHLO client.example
S: 250-mail.example greets client.example
S: 250 STARTTLS

Upgrading with STARTTLS

RFC 3207 defines STARTTLS, which upgrades a plaintext connection to TLS in place. In Go this maps cleanly onto wrapping the existing connection with crypto/tls once the client asks for it.

Authentication

RFC 4954 adds the AUTH command. I implemented basic mechanisms so the server only relays mail for authenticated senders — a small change that makes the difference between a demo and something you would not be embarrassed to run.