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.
Things to keep in mind:
I have a better method for recovery of the mbox. Most mail clients will have a way to import an mbox. Mine does.
https://support.apple.com/kb/PH14911?locale=en_US
Seeing as I use Google for my primary MX, and they keep copies of my delivered mail, I would also use Google Takeout. This would export my delivered mail to an mbox, and I could then import the mbox.