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

vim /etc/postfix/main.cf

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 (128.199.108.77)
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