As someone who's spent a lot of time and money trying to solve this problem, I will say that the biggest impediment is usability, and the largest technical hurd

As someone who's spent a lot of time and money trying to solve this problem, I will say that the biggest impediment is usability, and the largest technical hurdle on that front is key management (general users will never, ever take the time to understand what they're supposed to do with their keys). Beyond those problems, though, are the other problems mentioned elsewhere on this page; the inherent weaknesses in email that PGP can't circumvent (specifically the meta-data leaks--who is sending messages to whom). I think Adam Langley's Pond[0] is the most interesting candidate for an email replacement, although it's not yet ready for prime-time (and even further from a level of usability that could reach mass appeal). [0] https://pond.imperialviolet.org https://pond.imperialviolet.org