Apple Mail hides mail received on multiple addresses
Many common Apple mail issues can be fixed by rebuilding the Envelope Index.

MAKE SURE YOU HAVE CREATED A BACKUP, FIRST. YOU HAVE BEEN WARNED!


Quit mail.app
Hold down the Option key, then in the Finder choose Go > Library.
In Macintosh HD/Users/yourname/Library/Mail/V2/MailData, move all files starting with “Envelope Index,” such as Envelope Index or Envelope Index-shm to your Desktop.
Launch Mail and it will build new Envelope Index files -- this step could take a long time.