Postfix in a Chroot Cannot See Your MySQL Socket — Why Mail Stopped Both Ways
On a self-hosted grommunio server, no mail moved in either direction for weeks, not even between two mailboxes of the same domain. The log blamed a MySQL lookup. postmap -q as root answered perfectly. That contradiction is the whole diagnosis: the Postfix daemons were running in a chroot where the MySQL socket does not exist. Here is how to prove it and the one-line fix.