Hi Stephen,

Does it work if you delete the Maildir/ on the destination first and then initiate the backup?

I ran into this problem recently - see my post on 21 Feb.  Looks very similar if not the same root cause.

I've seen in a previous post, that the fact to do an RSYNC might break the  index.. So, I've heard that this is not recommended. that's the reason why I decided to find a way to do a "clean" backup and be able to come back online if needed.

So, do you use an RSYNC and in case of restoring the mailbox, do you do a  simple

 doveadm index -u USERx INBOX