Keep two IMAP servers in Sync

[Longman]

What's the easiest way to have two IMAP servers with an identical set of
email on each?

imapsync ? rsync ? neither?

Basically I'd like to have a central IMAP server accessible via the net
but sadly I have to go with it being a box on the internal net (due to
bandwidth and disk space reasons). This is fine for 99% of the time but
if we need to move to another building (in case of fire) I'd need a
duplicate box at the other location with all the email (unless I stick
the box in the boot of a car which isn't a viable option!).

What would be a good way of solving this kind of prob?