Email Fundamentals¶

These concepts apply to every mail system, whether it's Postfix, Exim, Zimbra, Exchange or a hosted provider. Read them once and every log line and configuration setting in the rest of the mail section has an obvious reason to exist.

1. Email Architecture covers the MUA/MTA/MDA roles, store-and-forward, envelope vs headers, and the ports you'll see.
2. The SMTP Protocol walks through a raw SMTP session line by line, response codes, STARTTLS vs implicit TLS, AUTH, and bounces.
3. DNS and Mail Routing covers MX, PTR and FCrDNS, relaying vs final delivery, smart hosts, and split-horizon DNS.
4. Email Authentication and Anti-Spam covers SPF, DKIM, DMARC, MTA-STS/DANE, DNSBLs, greylisting, content filtering, and current bulk-sender requirements.
5. Mailbox Formats and Delivery covers mbox vs Maildir, local vs virtual mailboxes, aliases, and handing off to Dovecot over LMTP.