We're not an edu, we're a nonprofit but I'm gonna chime in anyhow
First off, if Netbackup isn't working for you, check out Bacula. I've been using it for some time now and I love the #&@% out of it.
I highly recommend checking out the FOSS Backup guide in the Zimbra WIKI. Some of the info is a tad outdated but you can piece together a good solution for your needs.
Open Source Edition Backup Procedure - Zimbra :: Wiki
Basically what we do is keep it very simple:
(Our Zimbra server is hosted off site, btw but we still own the box. So we're fully responsible for our own backups.)
1: Stop Zimbra services
2: Kill leftover processes run by zimbra user
3: Use rsync to take a incremental copy of the /opt/zimbra folder
4: Restart Zimbra services
5: Make a gzipped tar of the backup folder
6: Transfer the gzipped tar file via SFTP to our main backup server on site to be written to tape.
Zimbra is never down for very long, usually only a few minutes. Transferring the tar is time consuming, but we do it only once per day. (Our backups are for disaster recovery, not archiving.) Sometime in the future I plan to implement LVM on the ZCS server to eliminate the need to shut down, but that also makes backups more complicated.
Cheers,
-n8