Demaestro - 9:01 pm on Jul 5, 2013 (gmt 0)
Do you have physical access to the servers?
You can get these hard drive replication units and hook it up to the server with a sata cable.
What it does is you take your drive out of the server and mount it in this unit, then you mount a 2nd drive into the backup slot. You connect the unit back to the PC with a sata cable and as data is written to your drive it gets written to the backup as well. If the main one dies you simply pull the backup and swap it for the dead drive.
I have seen some with 4 slots for extra redundancy. The advantage here is the replication hardware has the software built in so no processing power or bandwidth is used by your server to do that backup.
This all requires you can access the servers though...
If you can't set that up something I have seen done is you set up an SVN repository somewhere. Then create a bash script that does SVN-ADD, then SVN-COMMIT on the mail folders. That will push new files and versions to your SVN. Point a cron task at that script and have it run nightly or whatever time interval you wish.
Not the most elegant solution but it works and isn't hard to setup.