Encrypted email has been around and built into email clients by default forever and nobody uses it. Even just signing emails gets quizzical responses every now from people I send to (why do all your emails have a weird attachment?). The mere need give decryption keys to your contacts by a back channel has been sadly enough to push it over the edge of usability. People prefer total lack of security to even the most minor inconvenience.

It's sobering and worth deep contemplation by anyone thinking about solving this issue.