Could anyone offer some insight into running zcs alongside an existing email server?

Here is my situation:
I current run Kerio Mailserver on a Windows XP computer behind my firewall. I have a single static IP address. Kerio runs all standard mail protocols on the standard ports and any incoming email that hits the firewall is pushed to the internal IP address via the firewall.

I would like to have a live test installation of zcs running that any user could opt to use in lieu of Kerio without killing it - i.e.: duplicate or mirror the email. I'm not so concerned about calendaring or contacts. I just want live email on both systems.

I have zcs up and running in a vmware virtual machine (ubuntu), but there has been much difficulty getting imap accounts from Kerio to work in zcs. I assume this is mostly a zcs problem, but I haven't been able to work it out. I've started a thread about it in the administrators forum.

Anyway, can someone direct me how I might go about doing this?