<i>The only way I know of preventing the capability of post-reception editing is to have the sender and the person who is receiving the message obtain a digital certificate that is recognized by Microsoft.
Identrust, ORC, and Versign offer these kinds of keys but they are not cheap.
If it were a legal issue, maybe it would be possible for Exchange to keep a copy of the received messages in initial form as a comparison point should someone be suspected of editing an email.
<b>You can use the Journaling feature of Microsoft Exchange to get a copy of every incoming and outgoing message prior to the message being delivered to the recipient’s mailbox. This additional copy of the message is stored separately so the user cannot access it or make changes to it. This requires a lot of storage but it will provide the legal protection that you’re looking for.</b>