Can email be legally protected from transmission to third parties?

I have contacted a few people who, in their signature, they establish that their email may not be redistributed or printed for a third party under legal repercussions. Is this actually legal? If so, can they actually sue should the message be retransmitted to another party?

Can email be legally protected from transmission to third parties?
An e-mail, like a written letter or any other written work is automatically protected under copyright law. The author of the e-mail holds the copyright to the e-mail and can dictate how the e-mail is used, posted, published, or distributed, with the exception of special rules allowed under the Fair Usage clause.