A lot of servers don’t need to receive mail, as people nowadays use services like Gmail.
It is simple as changing the inet_interfaces directive in main.cnf from all to 127.0.0.1
inet_interfaces = 127.0.0.1
You can verify it by testing with nmap, it should say it is a closed port
nmap -p 25 domain.com Starting Nmap 6.40 ( http://nmap.org ) at 2016-01-28 22:55 EET Nmap scan report for embodyactive.net.au (184.108.40.206) Host is up (0.37s latency). PORT STATE SERVICE 25/tcp closed smtp Nmap done: 1 IP address (1 host up) scanned in 0.79 seconds