Looking in my logs, I noticed that my backups were failing with the error:
ERROR: Exceptions occurred during the run! ERROR: Server SSL fingerprint 'XXXXXXXXXXXXX...' for hostname 'mail.messagingengine.com' does not match configured fingerprint. Please verify and set 'cert_fingerprint' accordingly if not set yet.
I had to spend a couple of minutes figuring out how to extract the fingerprint from the certificate (using this useful answer on Stackoverflow) so to help anyone who is using Fastmail.fm for email along with offlineimap for backup, you can just add the following to your
~/.offlineimaprc to get back up and running.
... cert_fingerprint = 958b1601563aef92607a41eb5bad22e3cace8431 ...