Why is my boss not getting original customer e-mail if a colleague responds to it first?

Question

1. Boss and colleague are recipients of the same e-mail sent by customer with PO attachment.
2. Colleague responds using "Reply To All"
3. Boss does not receive original e-mail (with PO attachment), but only receives colleagues' response e-mail.
4. If a Search is performed, the e-mail shows up but not in the Inbox, the subject line is no longer in bold, meaning it has been read. (incorrect, my boss had never seen the customer e-mail, only the response from the colleague.
5. WHY ISN'T THIS SHOWING UP IN MY BOSSES' E-MAIL INBOX????