Well, I’m having the same problem. I have already backed up and reinstalled the client. When I copy the backup mail_data.dat I log in again on the client and it shows the error. Not being able to repair or recover the backup. Error:
System.Data.SQLite.SQLiteException (0x80004005): database disk image is malformed
If the database is malformed, backup is just going to result in a malformed backup, which is useless.
Are you using IMAP? If so, all your emails will be on the server, so you can delete the local database. First make a backup of eM Client and then close it and delete the “C:\Users\username\AppData\Roaming\eM Client” folder. You may have to change folder options in Explorer to show hidden folders so that you can navigate to that directory.
Now open eM Client and add your account. After a sync with the server your should end up with an error free database.