Here is what I would like to do. I have a domain called foo.com and that is setup as the main domain on my zimbra server. I also have a domain entitled exchange.foo.com. the exchange.foo.com domain is managed by exchange. Some of my users are hosted by zimbra and some by the exchange server. In my current mail solution I create alias for the exchange users on the postfix box and use a transport relay to send the exchange domain to the correct server. However when I am unable to do this in zimbra. I do not want zimbra to forward all foo.com mail to exchange.foo.com mail server, but only certain users. How do i do that? Below is a little chart that might illustrate this a bit better

Mail from Internet-> Address to Mikey@foo.com->Mikey, has a zimbra mailbox-> zimbra accepts and sends to his box.

mail from internet->address to Keith@foo.com ->Keith has an exchange box, as determined from an alias, ->zimbra forwards to exchange.foo.com Mail server.