A variant of this is the massive disclaimer on corporate emails, that 'binds' you to ridiculous levels of action if you receive an errant mail from them. Of course, it's not enforceable, as it's a one-sided contract - if you never agreed to anything, then they can't (morally, and usually legally as I understand it) force your hand. (IANAL)

Someone explained this to me by saying "If my email signature said 'by receiving this email, you agree to pay me $5', would that be considered a legal contract?"