I just had to sort out the AirSync problem. (I found the solution to edit the registry which seems to have worked)
But now when I close eMClient is says there are 3 emails in the Outbox, but its empty.
These are probably test emails I was trying to send when I had the Airsync problems.
Of course I tried ‘Send & Close’ and ‘Send all messages’ etc
No errors come up in ‘Operations’
If the message is not in Local Folders then first make a backup via “Menu / Backup”. You can see when the backup is complete in “Show Operations” via the dropdown on the right of Refresh.
Then “close eM Client” and try running the manual repair on the database as per below links.
To repair eM Client for Windows see @Gary link below.