I have been doing something somewhat similar to provide a backup for my mail server (I get mail delivered directly by SMTP to a postfix server on a home machine.)
I have simply duplicated the postfix setup on the two machines, then I change the port forwarding in my router to deliver it to either the main (default) server or the backup server. This is simple and not really what you're after. However I also synchronise the whole of my mail client software (mutt in my case) between the two systems using syncthing. This means that wherever the mail gets delivered to by postfix (i.e. either backup or main server) it gets duplicated to the other system by syncthing. This works perfectly, I can read mail on either system using mutt and everything looks identical and keeps in step. Syncthing is a very clever bit of software. -- Chris Green ยท
