We recently had a harddisk failure on our mail server (debian/postfix/courier) so I'm researching a way to build redundancy.
The first way I found was to setup multiple mx Records in the DNS Server so that the second Mailserver jumps in when the first fails. While this is easy to build it's not what I have in mind for redundancy.
The second one is to use a third box which provides a NFS share where the Mailboxes are stored. This isn't really an option, because it provides a single point of failure like a standalone Mailserver.
I would like to have a way to synchronize the two servers, so that, if the first Mailserver fails, the enduser doesn't notice there had been a failure. (Mails still in the IMAP Mailbox etc.)