Just consider:

    [log in to unmask]
    [log in to unmask]
    [log in to unmask]
    (etc)

Same  username.  Same domain,  except  for  the least  significant  part.
Totally different  user. It would  be a  total disaster if  mail software
were to  consider these addresses  to be the same.  This is why  the mail
standards say that these addresses are to be treated as totally different
people. For all you know, they aren't even in the same country!

  Eric