We are planning on having a backup/spare server and doign RSYNCS to it, but that may not be acceptable if you don't the possibility of loosing any mail.
We are using drbd and it works very well. Combined with Heartbeat and some APC powerswitches it's foolproof at fail-overs. Use a direct gigabit link between the servers using 9000 frames bytes proves to be enough to keep everything 100% in-sync. The additional mbox'es are placed in the users home directories, also high available NFSv3 servers using drbd. We have more then 20TB realtime syncronized using drbd 0.7.22; so far never lost a single bit. Use the seperate partition for meta- data so you can mount a 'none-drbdified' partition at anytime if troubles would emerge (never had to use it but better to be safe then sorry).
Our golden combination consists off: Beefy hardware with Areca 1260 raid controllers RedHat enterprise 4U4 Drbd 0.7.22 Heartbeat 1.2.4 bunch of scripts for the fail-over to control the power switches ( down means really go down using the stonith feature )
Goodluck, Leroy