If I understand correctly, zimbra-proxy (nginx) is exactly what you are looking for.
You just have to setup one (or several, with LB features if you want or just round-robin) zimbra-proxy server.
Then configure the mailbox servers to tell them there's a zimbra-proxy (a couple zmprov commands).
When a user will connect the zimbra-proxy server, ngnix will automagically proxify the request to the correct mailbox server (and do a bit of caching, that can be very interesting in IMAP mode). |