Only add emailed collaborators if we have a positive match on delivered to
email.

This is necessary to eliminate possible loop when an email is sent to a
virtual/aliased address. Not all mail servers add delivered-to header!
Merge pull request #604 from protich/feature/collaborators+aliases

Only add emailed collaborators if we have a positive match

Reviewed-By: Jared Hancock <jared@osticket.com>