https://techcrunch.com/2018/07/26/protonmail-now-supports-pgp-encryption-with-other-clients/

ProtonMail is arguably the easiest way to send end-to-end encrypted emails. But encryption only works by default with other ProtonMail users. The company is adding full PGP support so that you can send and receive encrypted emails with people who use other apps and services.

ProtonMail is pretty much like iMessage or WhatsApp, but for email. All communications between two users are seamlessly encrypted. It’s transparent for the end user as you don’t need to manage encryption keys yourself.