Postfix Help Needed - USAK Diplomacy Judge

OK, I'm setting up a Diplomacy judge. Everything is going well except
mail. My ".forward" file is, according to some judge-maint members,
correct. I have to pull mail off an external POP server, which I am
successfully doing with fetchmail. Mail enters the spool and appears to
go somewhere, but no game parameters are altered.

I know that this situation has to be pretty standard - A program hands
mail to Postfix, which pipes it to another command via the expression in
the ".forward" file. I just need to know how to configure Postfix to be
looking for mail coming from an internal program rather than via the
standard ports. The manual and all the websites happen to be a bit
overwhelming and the books I've read don't seem to be much clearer. I'm
not dense. What do I need to do or what information does someone need in
order to tell me what to do so that I can make this work? I've been on
it for 2 weeks and it's starting to get a little frustrating.

Chris