How to configure two external interfaces

I am using SuSE 8.1 with a DSL interface (dynamic IP address) and a dial up ISDN interface, with a fixed IP address. I want to block all of the incomming traffic on the DSL interface and allow some service (smtp) on the dialup interface. But in the SuSEfirewall2 config file there is no chance to configure the two external interfaces in a different manner. Is there another chance ? Rainer