Why email messege was deleted without being read ?

Issue

• Why Outlook receives a Delivery Receipt stating Your message was deleted without being read ?
• If user mailbox ( Postfix & Dovecot ) get full and sender from MS Exchange tried to send email to the fulled account then it get a bounced message Your message was deleted without being read instead of Quota exceeded (mailbox for user is full)

Your message

      To:  Postfix_User
      Subject:  Test
      Sent:  07/01/14 2:20 PM

was deleted without being read on 07/01/14 2:20 PM.