Outlook: missing items on the backend

[maxxer]

We’ve had a weird situation on a Zimbra install: user A using Outlook via AS, with a rather huge directory structure. I don’t know why they removed the Outlook account and started from scratch: they found out OLK wasn’t syncing some of these directories, so now mails that were only on the client are lost.
I’ve tried searching through Zimbra with some keywords but I was unable to find these emails, which were certainly originated on the client and moved in these out of sync folders.

Now, while we try to recover those lost emails, I’d like to understand how this happened, to avoid it in the future.
I made some tests and the only problem I was able to find is the problem which does not update directory structure. But even in this situation, even if the folder is misplaced the mails inside are still synced between client and server.

In the case a folder has 5 emails on the server, while on the client there are more than 50. How can a client have stopped syncing a folder previously known?

thanks