If you ever see the following in your logs,

warning: mail_queue_enter: create file maildrop/636773.1040: Permission denied

It measn postfix is not running, and you have problems!

Running
postfix check

will likely result in:

postsuper: fatal: scan_dir_push: open directory defer: Permission denied

What this means is that the permissions for postfix dirs are all screwed, most likely after a server restart (that’s when it normally happens for me at least).

You can confirm this by doing:

postfix start

and observing:

“postfix” postsuper: fatal: scan_dir_push:
open directory defer: Permission denied

To solve, run:

postfix set-permissions

You may still have to make minor permission changes, to change owner from root to postfix for the /var/lib/postfix files.