Just paid $50 to raise a ticket to be told:
The blank items indicate that the users mailbox.cfg file became corrupt for
their inbox. This can be corrected by performing the following steps:
1. Stop the SmarterMail service and ensure mailservice.exe terminates
2. Navigate to the users inbox folder, for example
3. Remove the mailbox.cfg file.
4. Start the SmarterMail service.
5. Sign in as the user, and there should no longer be any blank
GREAT! if only I had been told this on here as I know SM staff check these...