nicholas@mordor:~$ 
nicholas@mordor:~$ sudo lsof -i :25 
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
master  6537 root   13u  IPv4  41622      0t0  TCP localhost:smtp (LISTEN)
master  6537 root   14u  IPv6  41623      0t0  TCP ip6-localhost:smtp (LISTEN)
nicholas@mordor:~$ 
nicholas@mordor:~$ readlink -f /proc/pid/6537
nicholas@mordor:~$