So if you do the following steps it says the backup is corrupt?
To restore your previously backed-up Windows Mail email:
From the Tools menu, select Accounts.
Select your PO mailbox.
Click Import.
Locate the Mail Backup folder on your storage drive (location you previously backed up to).
Select the mail account that you want to import, and then click Open.
Repeat these steps for each mail account that you want to import.
Click Close.
Result: Your backed-up mail has been restored.