Email sent with a display name in the "MAIL FROM:" field of the SMTP conversation is passed to the downstream mail server and appears as if it was sent from an internal user. The mail client does not show the full email address so the end user may believe it is a from the internal user.
This is a specification of the RFC's for the SMTP protocol.
How the sender is displayed is a function of the email client, for instance Outlook. Messaging Gateway does not currently have the functionality to verify display names.
The current solution is found on a Microsoft technet blog post.
Other customers have found success using this information: