You cannot rope someone into a binding legal agreement without
their consent simply by e-mailing them. Those disclaimers at the
end of emails only have weight (legal or otherwise) if all
participants/recipients of the e-mail have explicitly agreed to
abide by such disclaimers. Common examples include: Internal
e-mails of a company where this is company policy; As a result
of a preexisting contractual agreement;
There are plenty of other reasons why it might be illegal to
reveal the contents of an e-mail (e.g. sensitive information)
but the "weight" there comes from existing laws, not any sort of
disclaimer.
The point of the disclaimers would be two-fold:
To remind people for whom the disclaimer actually applies that
the have to abide by it.
To scare people people into not releasing the contents of the
e-mail when they actually can.
In other words, they are mostly useless.