Need a Catch-All email domain

I'm running OS X Server 10.3.9. I host several web sites and handle email for those web sites. The built in controls in the Server Admin app have always been enough for me so I've never messed around with postfix at all. Email is not domain specific but other than that, it was worked out fine. But no more. I have a new thing I'm working on, and I need to make it so any email @domain5.com gets forwarded to one specific email address, while all the other email for all the other domains functions normally. I know that with the server admin app, you can easily make a catch all to grab all messages not sent to any existing user on the server, but thats not what I need. I need EVERYTHING at domain5.com (not the real domain) to get forwarded to a specific email address, even if the user name already exists on my server and that user already gets mail on my server (under another domain, although i know domains don't actually count by default for OS X Server). I'm repeating myself because I'm trying to be super clear on exactly what it is I'm trying to do. I even still want to be able to manage the rest of the mail server using Server Admin and Workgroup Manager with out it interfering with this modification. Once I'm done typing this up, i'm going to head over to postfix.org and see what I can uncover there. But I figured I'd post here first incase anyone had experience with this specifically on OS X Server.