OSX Server and mail setup for multiple domains

I've recently installed server on my old dual 500, works fine. Web is up serving multiple domains fine, so I decide to try and bring email services online. I can serve email for the primary domain just fine, the problem I have is if UserA doesn't have an account physically on the server, I can't seem to get postfix and friends to forward the email onto usera@ where.ever.net. Any ideas/hints?