I have read that zimbra wiki post about not using NFS, but I also read about other users here using it with a Netapp storage, that should have a robust NFS implementation being what they do for a living ... maybe that the NFS implementation on Mac OS X Server 10.5.8_x86 is not so robust.
What are we supposed to do with such a storage if we can't use it for a mailbox server?
What about iSCSI?
I know that is not natively supported by Mac OS X, but there is a 3rd-party driver here:
Studio Network Solutions - Download iSCSI Initiator for Mac OS X
that I tested once on our Xserve but its performances didn't look better that those of Xsan.