I don't think verification of sender is the problem (see PGP), it's that email was built such that anyone can contact anyone without setup ahead of time. I can give my address to some guy I meet at a conference and he can email me later with no issues, we don't need to agree ahead of time that we know who each other are. If he uses PGP, I need to accept the first communication is him, which means anyone can send me anything claiming to be anyone. Unfortunately, as soon as you solve this part of the issue with technology, I think you've just reinvented social media.
I wasn't trying to discuss whether or not PGP is a good or widely used technology. My point was that even if something like it were widely adopted and made seamless, it would not be able to solve the problem of bootstrapping the verification connections without eventually become social media itself.
