Email receipt confirmation

I hope never. This is such a privacy violation. It is no one's business whether I received an email or if I read an email. If I want you to know I will email you back indicating I received and read your email. Make sure you return a receipt email indicating that you received my receipt email. I will then return a I received your email receipt email to your I received your email receipt. See any problems here?

Not many email providers or apps actually support DSNs or read receipts, and in any event they are unreliable at best because there are too many places at the server or client level where they are either ignored or blocked.

The only system I know of that ever effectively did delivery/read receipts was Lotus Notes.