When you try to delete a data file in Microsoft Outlook, you get this error:
Microsoft Office Outlook
You may not delete your default data file. You must select a different data file as your default delivery location before deleting the current one. Go to Tools / Email Accounts to select a different delivery location.
Basically this error is saying that you may not delete the data file you are trying to delete, because it’s your default one that mail is delivered to. So to fix this, you need to make another data file the default before you can delete this one.
To fix this, in Control Panel -> Mail -> E-mail Accounts, View or change existing e-mail accounts, then change the drop down box (Deliver new e-mail to the following location) to point to the new data file you created. Once you’ve done this, you can then go back and delete the other one.