If you are not emailing from your VPS, you may find that you want to block access to port 25. This can help prevent spam from being sent from your server.
The steps may very slightly depending on the Linux distro that you are using, however the general steps will be the same. In this example we will use CentOS.
Step 1: Add iptables rules to Drop port 25
iptables -A INPUT -p tcp --dport 25 -j DROP
iptables -A OUTPUT -p tcp --dport 25 -j DROP
Step 2: Save iptables rules
service iptables save