I’ve taken to the concept of sending a copy of each incoming email to a second server, for backup. This could be achieved by an alias. Store them in mbox format. Use newsyslog to rotate it over time.
UPDATE: this strategy is mentioned in Scenic BGP Route | TechSNAP 137 at about 0:47:30.
As part of disaster recovery, my Maildir would be restored from backup. The emails which arrived after the backup would be replayed through procmail / maildrop.
I haven’t implemented it yet, but I’m planning to do so when I move my existing IMAP server into a FreeBSD jail.
For more information about this, be sure to read Mail backups using newsyslog.conf for rotation.