This is explained here:
http://www.readnotify.com/readnotify/about.asp[
^].
All you need it to add the specified string to the "To" header. Here, your technical problems ends. The rest of it is their proprietary technology. You are not doing anything essential yourself; you need to enroll sign-in for the service. If you have further questions, better ask the representatives of that company.
So, that request was almost off-topic.
—SA