[dragonwriter]

> Frankly I've never understood why email doesn't work this way

Because internet email is mostly designed as a highly fault-tolerant low-trust network, so the basic design is unauthenticated forwarding, so once its sent, there's no generally-applicable way to prove that you are the sender to ask for it to be deleted. Inside something like a particular Exchange server, email can be implemented that way (and basically is), and that works because its a single centralized database with client authentication.