Email confidential notices

I’ve noticed an increasing number of confidentiality notices in e-mail messages I receive. The last 3 companies I have worked for had some kind of agreement attached to each e-mail and most of the clients I work with do as well. Now I see a lot of personal e-mail from friends that have these agreements at the end. I would like to know how legal and binding are these agreements? By law can I really not show or share the message with anyone or is this just someone’s way to CYA?