I'm not aware of emails being fully tested, but I would presume that it follows a similar line to hard copy correspondence. The addressed recipient owns the contents of the letter, not the sender IIRC.
The main issue is then on how far you would have to go to protect or hide the identity of the sender. Largely (again IIRC) naming the individual or even organisation might be ok in some circumstances, but you may have to hide specific contact details and details of anyone cc'd etc.
Plus you'll see this everywhere, disgruntled people publishing emails received (executives of FIFA), etc. No action taken, because as far as I'm concerned no action can be taken.
So my guess is that irrespective of whatever disclaimer is put on the bottom of emails, as the recipient, I can forward it, print it and publish it to whoever I want.
And that's it. In a business capacity, if you want to send confidential or potentially compromising information, do not send it by email.