Email Spoofing on Exchange 2003/2007

Here’s my challange.
Company Background
We offer a HR recruiting solution that folks can use to post job/reply to canidates.

The current environment runs Post.Office to spoof client emails. I?ve been tasked with finding a way to make this process work with Exchange 2003 or 2007. I?ve been cursing the net for the last hour or so looking for a way to do this, but have yet to come up with a solution. I was wondering if anyone has setup email spoofing using Exchange in the past?