Email clients don't have to honor read receipts. In fact, apps such as Outlook, have an option where you can turn it off. And I've use a Lotus Notes macro that removed them.
Yea, and I doubt that the footers lawyers append to every email mean much in terms of disclosure, but that doesn't stop law firms from trying it anyway.
I think read receipts fall into the category of "good faith effort", and not much more.