Seeking Advice on email

I'm running a redhat 9 system using webmin and i have postfix installed on it. It's behind a firewall and connected to a IIS server.

In order to get mail out of that box i have to establish some kind of proxy server on the windows machine to handle email originating on the linux box and have the windows machine forward incomming and outgoing mail.

I have freeproxy running on the IIS box, does anyone know how i could configure the windows box to handle this? Or perhaps suggest another approach?