Intermittent Fail to Receive Email

I'm experiencing an odd situation with one of my users.  An account from a specific external domain will intermittently fail to arrive.  In today's case she was copied on the email along with another of my users.  He was able to receive the email and she was not.  There is no error or postmaster message in her email.  There is nothing in her Junk or Deleted Items.  She was able to see the message when my other user forwards it to her.

I don't understand how one person in my building will receive and another won't.  I have no errors so no clues to start off with.