postconf
NetworkPackage: postfix
Reads and changes Postfix's configuration. postconf -n prints only what differs from the defaults — the view you need when working out why mail is not leaving.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
postconf -n- Shows only the parameters explicitly set in
main.cf, without the hundreds of defaults plainpostconfprints. The shortest way to see what an installation actually configured.