i don't think i can help you out the the forwarding it onto your windows server, but i've setup a how-to make, install and configure a e-mail server with a virus scanner on fedora core 3.
just as another option if you can't get this idea working, you can setup your mail server on linux and still have the same username/password as you have on your windows domain. using winbind to authenticate against ADS.
i've got a how-to guide here
there is also a script i'm working on that should make the joining linux to your domain simple as 1, 2, 3
if you do decide to go this route, tell me how it goes, i'm looking for feed back on my scripts see if they have any problems will them.