Two External Networks: Workaround?

Good morning list:

It has been on this list, that two external interfaces are not going to
work with ISA.

However, in our special case, do you think the following workaround is
possible:

We have one internal network (192.168.10.0), one external dialup-DSL with
dynamic IP. So far so good. Since we need to receive our mail from
external, we have another router to a low-speed Connection (192.168.67.0),
that routes to the external provider.

Can we either:

1) declare the 192.168.67... network external, but provide a static route
to the IP of our external mail-router (this is a fixed IP) ?

2) declare the 192.168.67... network internal and do port-forwarding (to
ISA or directly to the mailserver) on the router?

Thanks for any feedback...

Stefan

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