Blocking SMTP connections

If a VPS user has been found to be spamming other users, you can block them temporarily from accessing SMTP ports by issuing commands similar to the following on the node that their VPS is situated on:

 iptables -t mangle -I POSTROUTING -s <IP ADDRESS> -p tcp --source-port 25 -j DROP
 iptables -t mangle -I POSTROUTING -s <IP ADDRESS> -p tcp --destination-port 25 -j DROP

To unblock, just use the -D rules:

 iptables -t mangle -D POSTROUTING -s <IP ADDRESS> -p tcp --source-port 25 -j DROP
 iptables -t mangle -D POSTROUTING -s <IP ADDRESS> -p tcp --destination-port 25 -j DROP