My Mail app (MacOS Catalina 10.15.3) displays phantom unread messages
- Quit Mail if it’s open.
- In
~/Library/Mail/V7/MailData, delete any file that begins with “Envelope Index,” such as Envelope Index or Envelope Index-shm. - Your home Library folder is hidden by default. To display it, choose Finder > “Go to Folder” and then enter “
~/Library.” - Open Mail.
- Mail creates new Envelope Index files. This process may take a few minutes, depending on how many messages Mail is reindexing.
It will fix your issue.