Funnily enough I'm just trying a similar thing myself having failed to get DRBD running for an HA setup.
I've mounted the store and index folders on a test server via NFS but I can't seem to import anything to it at the moment, I keep getting the message
Although I did read this thread (http://www.zimbra.com/forums/install...ity-setup.html) which suggests NFS is a "bad, bad, bad idea".
ERROR: service.FAILURE (system failure: Error While Delivering Message)
EDIT: I have the NFS writing ok now, turned out I was mounting the folders incorrectly.