A number of users are experiencing problems with Windows Mail and
deleting messages from the Inbox and Outbox. The message will first
become unviewable in the reading pane and will then generate an error
when you try to delete it. This is usually caused by a corrupt
mailstore. To rebuild the mailstore (WindowsMail.MSMessageStore), follow
these steps:
1. Back up the email you wish to keep. While rebuilding the mailstore
shouldn't result in any loss of email, it is always better to be safe.
2. Close Windows Mail.
3. Click the Start Button and select Computer.
4. Navigate to C:\Users\your
username/AppData/Local/Microsoft/WindowsMail.
5. Delete WindowsMail.MSMessageStore.
6. Open the Backup directory.
7. Open the new directory.
8. Delete WindowsMail.MSMessageStore.
9. Open Windows Mail to rebuild the mailstore database.
You should now be able to view and delete the problematic message(s).
I found this on tech -recipes website,Hope it works,