Zimbra Cluster is the way if you need HA for your mailbox. I have setup 1+1 all-in-one cluster (50 users) and also 2+1 mailbox cluster (2000 users). FYI, the documentations available at the support portal help me through it. From my experience, the only thing that need further documentation are regarding zimbra SAN partitioning; i meant details of the space needed of all directories based on users/mailbox quota. So far, I haven't encountered any confusions/issues except the SAN partitioning part.
All of the above system running zimbra 4.5.4 with critical bug patched. I still haven't got the time to upgrade them to latest version. Will be glad to hear from those who have the 'cluster upgrade experience'.
