[exim] Redundant mail servers...

I would like to setup a redundant, robust mail server system using exim
and was wondering if this is possible.

The idea is to have one primary mail system that normally processes
everything, and a secondary fail-safe mail system that will seamlessly
take over when the primary server is down.

While a hotsync situation is preferable, I can imagine that it is more
costly. So in principle, a less elagant solution with scripts could be
possible, e.g. an hourly/daily snapshot backup with rysnc.

Any ideas and/or references on how to do this would greatly be appreciated.

Thanks in advance.

--
Kiffin Gish