4.1Configuring and Managing Postfix

Email works through a division of roles: sending (an MTA), receiving and delivering (an MDA), and retrieving (an MRA). Postfix fills the MTA role and is the most widely deployed mail transfer agent on Linux. Rather than memorizing settings, having a map of "which file decides what" is the shortcut for both real work and the exam.

4.1.1main.cf, master.cf, and aliases

• Config files live under /etc/postfix/. main.cf holds the daemon's runtime parameters (accepted domains, relay permissions, TLS settings, etc.) as one parameter per line. master.cf is a table defining which services (smtp, submission, pickup, ...) run as which processes.
• Redirect destinations via /etc/aliases (e.g., root: admin@example.com forwards mail addressed to root). After editing, you must run newaliases (or postalias) to rebuild the hashed database—without it, the change has no effect.
• Mail is not always delivered instantly—it passes through queues under /var/spool/postfix/ (incoming, active, deferred, hold, corrupt, etc.). Inspect or force-flush stuck mail with postqueue and postsuper.
• Runtime logs land under /var/log/ (often /var/log/maillog or /var/log/mail.log, or via journalctl on systemd-based systems), and are the first place to check for delivery or authentication failures.