Collaboration or integration with Mailpile [1] might be a useful direction to consider. They are doing interesting work on the backend, but as far as I am aware

Collaboration or integration with Mailpile [1] might be a useful direction to consider. They are doing interesting work on the backend, but as far as I am aware, there isn't much frontend present yet. [1]: https://github.com/pagekite/Mailpile https://github.com/pagekite/Mailpile