I implemented this on an invoicing app I make. The app also lets users create "estimates", "quotes", "proposals" and other documents which I do not use it on.<p>The reason I did this is because over the first decade after creating the app the users of the app kept telling me that clients would tell them "I didn't receive the email".<p>I've had clients do that to me as well, even before I made the app. One of them was a contractor for the U.S. federal government who owed me over $20k and was most certainly going to screw me. In that case, after sending the invoice with the US Postal service three times I sent it the forth time with a "Return Receipt" required. Even that didn't get my check in the mail from them because they lied again about receiving it until I informed them I had a signature saying they did get it.<p>Still, the only reason they paid me is because I shut down web based "Real Time Traffic Conditions Map" I'd created for the project a few days before their public debut of the app and refused to turn it back on until the check was in my hands.<p>I don't really consider this an invasion of privacy. All it is telling my users is rather or not their customers and clients opened the emailed invoice and it's little different than the "Return Receipt" service offered by the US Postal Service.<p>FWIW, it works with Gmail too.