How to Fix Wrong Unread Count in Smart Mailbox in MacOS 11 Big Sur

MacPaw:

Reindex your Apple Mailbox

1. Quit Apple Mail if the app is running.
2. Launch Finder and select Go from the Menu Bar.
3. Hold down the Option key. This will bring up a new item in the Go drop-down menu: Library. Select this option.
4. Select Mail > V5 (or the current version) > MailData. Locate and delete all the files that start with Envelope Index, such as “Envelope Index” or “Envelope Index-wal.”

Note: It’s a good idea to copy each file to your desktop before deleting. Launch Apple Mail. The next time you open Mail, it will build entirely new Envelope Index files. You may have to be patient—this process can take a while if you have a lot of messages for Mail to go through.

Now, everything should be working as normal! If so, you can get rid of the Envelope Index file copies from your desktop.