I said you can’t sync message notes between two installations of eM Client.
Restoring a backup is not syncing. It just restores eM Client to the state it was during the backup. If there were notes on messages then, they will be there after the restore, assuming the messages are still on the IMAP server.
Gotcha, @Gary. Sorry for misreading …
(I read you as emphasizing what eM Client can/cannot, so I was thinking “maybe I can trick/help eM Client by cloning it’s databases/files somehow” …)
So, I guess I will not try to sync files like these between computers, then, in ~/Library/Application Support/eM Client/<long profile identifier>/<email account identifier>:
Individual files from within the database are not transferable to another database. You will need to copy the whole database. That is not syncing. It is backing up on one and restoring to the other, which will complete overwrite what is on the destination.
So as I said, it is not possible to sync message notes between devices unless they both have the same account setup as EWS. Then the message notes will sync through the Exchange server. There is no other way.