Improve the experience for privacy-minded users.
Privacy mode: blocks tracking pixels and disables open/link tracking in outgoing messages. (Mutual privacy)
PGP/GPG email signing.
PGP/GPG encryption support.
Improved email headers/source viewer.
(Proposed) Show warnings above message when headers indicate DKIM/DMARC/SPF failures.
(Proposed) Keybase support.
(Proposed) Support traditional delivery receipts.