Which ports are postfix and dovecot listening on?

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.service​

Then, 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 -nl​

I discovered that something was listening on TCP 25. Then, I checked postfix service by running

systemctl status postfix.service​

It 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.

 

I don't know what was listening on 25 while postfix was dead ( do daemons die or do they become angels for a while? )
but you could configure monit the way it checks postfix pid for example
usually it's

Code:

check process postfix with pidfile /var/spool/postfix/pid/master.pid
start program = "/etc/init.d/postfix start"
stop program  = "/etc/init.d/postfix stop"
if cpu > 60% for 2 cycles then alert
if cpu > 80% for 5 cycles then restart
if 5 restarts within 5 cycles then timeout

maybe your watchdogs conflicted and cycled more than 5 times?