DrayTek 3300 Block SMTP WAN2

Can I block a port number on a certain WAN interface on the DrayTek 3300?

I have been looking through the firewall and port block settings but cannot find a way to stop our server sending SMTP through the 2nd WAN which is a dynamic IP and is therefore blocked by everyone.

Has anyone any idea how I can do this?

The firewall set-up seems to be entirely IP based which means that I have no means of differentiating between the WAN ports.

You can do this with load balancing policy in the Draytek 3300 by just making SMTP go to 100% on one WAN or another.

Under load balancing you will see the setting with no changes to firewall or filter settings.