Use netstat command to analyze the current open ports and the tasks/processes listening/using those ports. 
 
e.g netstat -nao | more 
Displays network connections including the protocol (TCP or UDP), the IP address, the ports listening, and the PID/process using port, one page at a time. 
 
 
For example, Domino is reporting that bindsock cannot give it port 25 for the SMTP process. Mostly like postfix is running on the server since an update. To confirm: 
$ sudo netstat -tuplen | grep 25 
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      0          26402      2084/master          
...    
tcp6       0      0 ::1:25                  :::*                    LISTEN      0          26403      2084/master 
 
$ sudo readlink -f /proc/2084/exe 
/usr/libexec/postfix/master 
 
$ sudo status postfix.service 
... running ... 
 
$ sudo stop postfix.service 
$ sudo disable postfix.service 
Removed symlink /etc/systemd/system/multi-user.target.wants/postfix.service. 
 
$ sudo netstat -tuplen | grep 25 
<nothing on port 25 returned > 
  
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