First off think of bind as allowing domain members to send mail to each other first.
Then and only then if the user is in another domain query the outside DNS (in your case win3k ip addresses would be used) for outside ip-addresses.
Check this excellent how to for Linux in this case the distro is Ubuntu but bind is bind:
Ubuntu 6.06 Server (Dapper Drake) Beginner's Install Guide - Zimbra :: Wiki
This should get you where you want to go.
Cheers,
Jonathan