You can use rsync, I believe. I have not used any other
backup scheme, available with zimbra or not, so take it
for what's it's worth!
Anyway,
Create a shell script to do something like the following,
then put it into cron to run in the wee hours of the morning:
/sbin/service zimbra stop
sleep 10
/usr/bin/rsync -avz /opt/zimbra username@someotherserver:/backups/zimbra
/sbin/service zimbra start
You create a user account on a remote server and then use openssl
to generate a key (er, long story there, google openssl), or you can
even use rsync to just put a copy of zimbra onto a local volume, like this:
/usr/sbin/rsync -avz /opt/zimbra /backups
This creates a complete copy daily, and it can be fully restored should
things go belly up without issue.
AFAIK, zimbra keeps everything, and I mean everything, you need
in the /opt/zimbra directory (pretty much, except for OS-specific stuff
like startup and shutdown scripts, but you can add that to the script
above).
Zimbra admins - please correct me if I'm wrong about this...