Block all Ports only open Postfix SMTP port

You could change your firewall (iptables) settings via webmin. Just go to your virtualmin start page, click on "webmin" on the upper left side - you will find your iptables-settings at "Networking" -> "Linux Firewall".

Depending on your current settings, you will be asked to set up your firewall or to store current iptables settings.

Afterwards you could use the "Add Rule" button below the listing of "INPUT"-rules to add a new rule. Just set up two roles:

• action: accept
• Destination: [EQUALS] your additional IP
• Network protocol: [EQUALS] TCP
• Destination port: [EQUALS] 25

Second rule:

• action: DROP
• Destination: [EQUALS] your additional IP

A better way would be to set "drop" as default action and open used ports manually - but that would depend on the use of your other IP-adress.

At least you have to check all rules and apply the current settings by using the button at the bottom of the page. Keep in mind, that wrong firewall settings might keep you out of your server/webmin.