We at Lavaboom use PGP manifests to hide most email metadata inside the envelope [0] [1]. Inspired by [2].
0: https://github.com/lavab/pgp-manifest-go
1: https://github.com/lavab/pgp-manifest-js
2: https://www.mailpile.is/blog/2014-11-21_To_PGP_MIME_Or_Not.h...