[clug] backing up a live mailserver

[Tony and Robyn Lewis]

We're running Dovecot IMAP on our mailserver, and we want to add a 
backup mailsever.  We'll have it as a secondary MX record, for when the 
primary goes down.

At the moment, it's a mental exercise, but the difficult part is that we 
want to keep the entire mailboxes synchronised, not just buffer incoming 
mail on the secondary.  I'm thinking of using Unison to help keep them 
in sync.  Done right (I hyphothesise) it should take care of 
sycnhronising when the primary goes down, and when it comes back up.

I would have thought that this would be a common problem faced, but 
can't readily find HOWTOs or recommendations.  In the collective wisdom 
of the CLUG hive mind, does this sound like the right approach?