For me, it remains a mystery what "read notification" actually should imply. Is it that the users mail client has downloaded the message, did the user read subject and maybe a preview, did he accidentally click on the message while navigating to another message, did he… You get the point.
Whenever you want to be sure that someone read and understood the message (and probably even started acting on it), then the only mechanism is to ask the reader for a reply notice: "Please act on it ASAP and let me know when you start by replying." And if it is urgent (or the recipient may be reluctant), call them on the phone.
If your goal is to be informed without the reader being aware, I consider this very unethical and (as I mentioned above) may have very little significance (and, depending on the way he reads the mail, may not even be triggered).
While I am very much a techie, technology alone can never solve social problems.