Here's the deal. I managed to get Ubuntu 7.10 server and Zimbra 5.04 to play nice this weekend. I had to use 7.10 instead of 6.06 due to Grub not loading correctly on my test box, but that's another story.
The current mail server is a Debian 4 box, running Exim, Courier-IMAP, Mailscanner, and PHPGroupware. It works, but the box is getting unreliable and PHPgroupware is really showing it's age.
I've got the Zimbra test system running on my network, nslookup and DNS are working, DNS on my main DNS server has MX records. All works, I think.
Is there a way to have Zimbra running side by side, mirroring my production server for specific users? would it be as easy as setting up users and accounts that point at my current mail server? Or is there a way to have mail split between both servers?