I have 3 kinds of software to monitor my public server. They are: Nagios v4 Monit v5.16 ISPConfig built-in monitor I stop the postfix service by running systemctl stop postfix.serviceThen, I waited for 10 minutes but none of the above monitors tried to start postfix service again! I then checked the listening ports by running netstat -nlI discovered that something was listening on TCP 25. Then, I checked postfix service by running systemctl status postfix.serviceIt showed that postfix is dead/inactive! Now my question is: on which ports should postfix listen? Which process was listening on TCP 25 while postfix is dead/inactive? My system is ISPConfig 3.1.7, Ubuntu 16.04.3 amd64.