Hello
Many different dsync issues have come up in this thread. Ill try to answer them as best as I can.
dsync backup -R The conclusion reached in the thread was correct. Instead of the backup option, doveadm import would be better suited for merging old mails into an existing mailbox.
Maildir + INBOX + backup/sync/replicate In the test scenarios where the INBOX on one side was to be completely removed, e.g. doveadm backup -R the dsync failed and nothing was synced to the target. This is because before moving the source mails to the mailbox, dsync cleans out the old ones ( -R preserves nothing) and in Maildir the INBOX can not be removed. This is a feature/not easily solvable, because in Maildir INBOX is different from other folders.
dsync replication / doveadm sync not working as expected. These came in pretty late in the thread and I did not get a full picture of what kind of setups and parameters were used. I suspect these might be a configuration issue. I think trying with different configurations and going through the documentation, such as it is, once more, is your best bet. Use -D and -v to make dsync more verbose, so you do not miss any error messages.
br, Teemu Huovila