Postfix: postfix/postdrop warning: mail_queue_enter: create file maildrop/N.N Permission denied
This happened to me when I limited the used systemd
to limit Apache to NoNewPrivileges=yes
Everything worked ok until a Cubemail (a webmail client) user
attempted to send a mail.
Cubemail used sendmail which used Postfix maildrop.
It needed to setgrp to become user maildrop to do its work.
But that wasn't allowed because of the setting.
So you cannot lockdown your webserver with that setting if Cubemail is in use.
Look for that that settings in:
Once its change to NoNewPrivileges=no do:
systemctl restart httpd