We're still in the fairly early stages of rolling out our Zimbra deployment, so we don't have that much data yet (< 100gb), but the Netbackup copy of the backup volume is already running out of window. We're at Netbackup 6.5, so a newer version isn't going to help you.
I've been thinking about ways to get rid of the Zimbra backups entirely - we use NetApp via iSCSI for all of our primary volumes and will use a Thumper running ZFS and iSCSI for the HSM storage. Both of these support snapshots. Now that Zimbra has a command-line tool to replay the redolog, it should be possible to restore a snapshot and then run the redolog against it to bring it up to a point in time. It's on my "to do" list to test this thoroughly in our dev environment next month. Of course, this only works as long as you ensure your redologs are stored separately so that they can't be taken out at the same time should you suffer a storage failure.
On the NetApp/Thumper side, the iSCSI LUNs are just files, so they're easy to send off to tape quickly if a spinning-disk backup isn't sufficient.
The Zimbra engineers indicated to me that they're looking at a mechanism to quiesce the server so that a snapshot can be taken, ensuring that what's on disk is consistent. A simple script could then signal zimbra and mysql to flush buffers and quiesce, then trigger snapshots on all storage, then restart the servers. That would be a lot quicker than actually shutting Zimbra down
Simon Fraser University