Recovery problem

When you click on the + sign a window pops up for you to add a name but if you click on the down arrow by the mailbox heading a list will pop up with all the mailbox names. Just pick the one(s) that you want back. Your old mail files can be found in your library. Open Finder, then GO with option key and select library. Then mail then V folder. You will see several folders with letters/numbers. Open those and you should find your backed up email folders. You can then export to desktop then import into mail. Any mail folder that was "on your Mac" should always be saved and restored by TM. Other emails can be backed up using a program like MailBackupX since TM no longer works for retrieving specific emails with Big Sur.

If you are referring to the mailbox folders in the left column in mail they may be hidden. Click on the + sign to the right next to your mail heading and then select the mail folder that you need.