An email-alternative over the Matrix protocol, called "matrix letters" (short: male), or the enhance version "encrypted matrix letters" (short: e-male), or the most sophisticated version "federated, encrypted matrix letters" (short: female). Let me know how much you hate the name :-)
Plain text mail works more or less fine, but encrypted mails with PGP failed to gain any traction and can easily be mis-used leak data. This happens even to technically well-versed users.