Lawyers and experts on internet policy say no court case has ever turned on the presence or absence of such an automatic e-mail footer in America, the most litigious of rich countries.
That's not strictly true.
Legal fights are like a chess match. In much the same way that a chess game doesn't turn on a single piece, a legal fight rarely turns on a single item. Instead, both sides struggle to get the better position. Most cases are settled out of court, and the better your position, the less you pay (or the more you receive).
A lot of what lawyers do appears absurd from our point of view, but it's all about position. For example, I used to work for a company called ISS where an employee (Mike Lynn) published trade secrets at a conference. These secrets were quickly picked up and mirrored on websites throughout the Internet. ISS sent out cease-and-desist letters to those sites, telling them to take down the content. This was absurd -- it was like trying to take pee out of the pool -- but it served a purpose. It demonstrated that ISS was willing to go to absurd lengths to protect its trade secrets, putting it in a stronger position the next time a trade secret case happens. Companies that do not protect their trade secrets are in a weak position if their employees steal the secrets.
Misdirected and forwarded emails are a common problem among law firms, such as in the Arcadis case. Arcadis was suing former employees who had left to start their own company. The lawyers for the employees accidentally sent secret documents to their former email addresses @arcadis.com. Apparently, this was due to the "autocomplete" feature of email programs that automatically fill in known email addresses.
The consequences were horrible -- for Arcadis. Even though it was the mistake of the lawyers on the other side, it was the recipient of those misdirected emails who suffered. The misdirected emails were forwarded to the in-house counsel, who then forwarded them to the law firm prosecuting the suit. This was improper -- the court ruled that Arcadis had to dismiss their law firm from the case, remove their general counsel from further involvement, and start the law suit all over again. And pay $40,000 to the former employees.
Did the case hinge on the fact that there was a notice on the bottom? Hard to say -- but it certainly put them in a stronger position.
Such notices serve other purposes. For example, let's say that the opposing side serves you with a subpoena for all your emails. Those that are "attorney-client privileged" are exempt from the subpoena. Marking them as such makes them easier to find on your email system and strip out before handing the email over.
I'm sure lawyers can tell me many more reasons for these notices. The point is: even though they make no sense to us, and do not impact us, it doesn't mean they aren't important to lawyers. They are toothless but not pointless.
So, what should we do about these notices? Well, my policy is to ignore them.
My position cannot get better by reading these notices -- but it can get a lot worse. I should get advice from my lawyer and not from these notices that imply all behavior "may be illegal". If I make a mistake, and go to court, they will try to claim that I should've known by reading the notice. It's better that I not have read the notice at all.
Legal notice: I am not a lawyer, therefore, this is not legal advice, and you should not follow it. On the other hand, if you misinterpreted this as advice and followed it, then you wouldn't be reading this notice anyway. My brain hurts.