Sadly, I haven't figured out a proper fully-redundant solution. Syncing mailboxes with e.g. Syncthing is way too fragile. Recently, I wanted to experiment with DBMail + CockroachDB cluster but haven't yet found time for that.
Never found any MRA that uses document-oriented database like RethinkDB; and GlusterFS is way too sensitive to high latencies - so DBMail+CRDB looks like the only readily accessible option sans of trying more "raw" programmable solutions like Salmon or Haraka and writing own storage backend.