MicroDot-II allowed me to "Request reciept", and it would be nice if SimpleMail supported that too. I'm not sure if this feature is supported by many email programs, so it isn't a priority or anything, but perhaps you could add it to the bottom of your "ideas" list?
I believe that I added this feature request. I think you can delete this request, as I don't think Read/Delivery Receipts are hardly supported now.