Q: How do I unlock an email

Try this:

1. Close the error message and Quit Mail
2. Open your Home Directory
3. Open your Library folder
4. Open your Mail folder
5. Select the file “Envelope Index” and if it there “Envelope Index-journal” and drag it to the trash
6. Open Mail
7. Apple Mail will ask if you would like to import messages say “Yes”

 Cheers, Tom