All emails flow into that folder first and sit there for a little bit then get pushed into the CFG/GRP files for the specific folder. Data can be in that folder and the user will still see those messages as if they were in the intended folder. User's can interact with those emails, flag them, and read them. We push these transparently into the GRP/CFG files. If you see these hanging around it could be that the user has deleted the folder that the messages used to be in. We do have cleaning that should clean these folders up and it makes sure messages are delivered.

Can you enable the Debug log,
20191228
In System Admin > Troubleshooting > Debug Log Ids.

Once that's enabled, log into the user, and then wait a little bit. After some time passes you should see that user's email somewhere in that log. Could you send this log to me?

Thanks for explaining Matt - I expected this folder to be used for something like this.

We've just moved (via webmail) several folders that were nested under a client's Inbox to the root level. These all moved fine, but after several minutes all these folders lost their contents (they're now empty)!

The MailProcessing cache folder contains just over 3,000 items.

Shall I sit and wait, or restore a backup?

I would first enable that log, log into the user, and then do a restore. You can send me the Log and the exact steps you did to put it into this state. I can try to reproduce on my side. Go ahead and restore those folders. If you can't restore we may be able to recover those MailProcessing items with a bit of rigging as long as they are kept, as is.

I've already enabled the log and logged into the webmail as a user.

Have you got instructions for performing the restore please?

I did take a complete backup of the user's folder before we bagan... so we can roll-back to this if need be.

It looks like these emails have now re-appeared!

Not sure what caused this to happen. Maybe it was the server reboot, or the couple of times the SM service crashed... or the Windows Updates that we've just installed.

Or, maybe, SM just needed some time to process the data in the MailProcessing folder.

Could the cleaning you have in place have fixed this? If so, when does this cleaning run and can I check the progress via logs?