Thats actual very interesting that Outlook has a built-in edit feature. Maybe its used for adding markup to a message for a reminder to yourself.

By the way, no email can actually be used to prove that someone said or communicated something. If you would like I can send you an email from any email address you would like, such as Bill.Gates@microsoft.com.
It is very easy to spoof message credentials since there is no built in security mechanism for them. Thats why you get untraceable spam.

Also, if a user really wanted to edit a message locally they could edit the binary that held the messages.