I believe cpGuard have already fixed this but there was an issue until recently where the cpGuard installation would install and enable the postfix service on the host o/s.
This would not initially cause issues but on reboot it would be impossible to start the Enhance email server due to the port conflict.
If you installed cpGuard recently, please log in as root and run:
systemctl disable postfix
systemctl stop postfix
Then if your Enhance mail server is offline:
docker start mtacd