You might need to study the legal notion of implied license. This has been tested multiple times in the courts already. Property vs. Copyright: Legally, the email content is not considered "property" in the traditional sense; rather, it is a literary work protected by copyright law upon creation. While you are the author and hold the copyright, the recipient receives a copy and typically gains an implied license to use, store, or share that content unless a prior agreement (like an NDA) restricts this.

You might need to speak to your lawyer.

I just did. I sent your post to him :)