I definitely have not seen this behavior before with the CRM for Outlook client. Now, if you open this appointment, click on the Developer tab and select Design this form. Once in here, click All Fields and select All Mail Fields. Does this appear here?
If you do not have the developer tab, in Outlook, click on File| Options| Customize Ribbon and check the box next to Developer on the right side.
Also, if you select any of the CRM tracked recipients, do their emails resolve to this external contact?
This sounds like a third party add-in that may have intervened here somewhere.