[prophesi]

Very true! I personally use both ProtonMail and Tutanota, and give my thanks to Proton for opening their OpenPGP library. I don't think the source to their servers is open, but that's not necessary since:

1) you can't verify they're actually running that code anyway

2) it's end-to-end encrypted so as long as the client-side code is indeed doing its part, your message could be sent to everyone on the planet without any fear of anyone figuring out its plaintext.