I've been looking for a way to show proof of delivery of virtual items and Whoreadme does the trick. Not only that, also the basic service (up to 20 emails/day) is free, and the paid extras are inexpensive. I'm very happy I've found this website, especially considering that alternative services like Rmail are incredibly expensive.
The amount of information the service provides is amazing: date and time of delivery, geographic location (with map), browser, system, IP and read duration!