Email (once abused) sucks because it was born when trust was implicit between both parties in a communication, plus the creators wouldn't even imagine that wonderful tool one day would be used for mass spamming, "419" scams and the like. Those days are long gone sadly, but the protocol is there in the open to be hardened and made more modern (just like NNTP could become a social networking killer if properly extended). Add basic security by default, write clients so conversation mode becomes the norm and you get a mail client that would be much much easier to push to other users. Example: https://wiki.gnome.org/Apps/Geary

100 times easier than reinventing the wheel, and we get the huge benefit of using well known open protocols and decentralized servers that can talk to each other, and are already there.