Only import collaborators when delivered-to email is known. This is
necessary to avoid adding internal virtual address as a collaborator.

Store delivered-to email ID - giving us the ability to reply FROM specific
email address as opposed to the department's email.
Merge pull request #537 from protich/feature/delivered-to-email

Only add collaborators if the system email in the To and Cc list was determined

Reviewed-By: Jared Hancock <jared@osticket.com>