Backup strategy for large mailbox stores

[Vincent Fox]

I suppose replication and snapshots are out of the question for you?

We run ZFS so snapshots are atomic and nearly instant.
Thus we keep 14 days of daily snaps in our production pool
for recovery purposes.  In our setup the total of all the snaps
is about a 50% overhead on the production data which seems
cheap to us dunno about you.  Helpdesk people can just run a
utility while talking with the user and instantly restore a particular
day of mail into a subfolder in the user mailbox.

An offsite replica with larger/slower storage and more snapshots
seems like it would fill most people's needs.  Dunno about your
data retention policy.  Ours does not require us keeping it for
years or anything so our mechanism are largely for disaster
recovery both individual and system.  We do run a weekly
netbackup full on the weekends but could probably dispense
with that if we ran into your problems.