If I can point you in the right direction, maybe use rsync (to sync the two mail stores?), or do a daily backup and restore of the DB if that is where you store your mail.
You have to setup a new MX record with your Name Server. Giving it a higher number then your primary server, it will then act as a backup. Here
is more information on MX records.
With Linux there are SO many ways to get the job done... Personally, I try the hard way first so I understand the efforts a replicator application is saving me, and know a little bit of what runs in the background if I need to fix it...