We are doing the same thing. I have a post in asking about the configuration rewriting that occurs when you restart Zimbra. However one of our clients is moving back to Exchange from Zimbra due to Outlook connector and mobile phone issues.
Basically you will be changing the configuration of Postfix to act as a mail gateway for those domains still using Exchange. You can do this with a combination of relay_domains, relay_recipient_maps, and transport_maps.
I believe the
The Postfix Home Page site has examples of a mail gateway.
I have it working (sans recipient auth) my problem is that the transport_maps parameter in main.cf is recreated each time postfix is restarted so I do not know how to make my changes stick.
Let me know if I can be of further assistance. I am no mail expert by any means (I am not even and admin) but I have gotten this work.