I found the answer. Not a good one.

"In Microsoft Outlook, only the sender's Display Name of the original message is added to the body when a message is forwarded or replied to. If the recipient wants to send mail to the original sender, only the Display Name is available. In a Microsoft Exchange Server organization, it is possible to look up the address in the global address list; however, if the original sender is outside of the Exchange Server organization and has no custom recipient entry, it may be impossible to find the e-mail address of the original sender."

Sorry.