No, currently that is not possible.
On the other hand whenever a user emails another user he gives his email address away, so does the user when he uses the public frontend instead. So I doubt that you have a privacy issue when a user submits his email address for that reason. You surely may have an issue when you are collecting these addresses and sell them or use them for ads etc.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer