If you’re troubleshooting problems with Apple Mail, where it either is hanging or is displaying ghost messages in search results, try this troubleshooting technique…
- Quit Mail
- Navigate to ~/Library/Mail
- Rename the file named “Envelope Index”, giving it a name something like “Envelope Index BU”
- Relaunch Mail, and click Continue when asked if you want to import your messages
- Wait for the process to complete
- If the problem goes away, delete the renamed Envelope Index.