The way Protonmail and co try to solve this problem is by sending the non-protonmail recipient a notification email, with "click here to read". From https://pr

The way Protonmail and co try to solve this problem is by sending the non-protonmail recipient a notification email, with "click here to read". From https://protonmail.com/security-details https://protonmail.com/security-details : "We support sending encrypted communication to non-ProtonMail users via symmetric encryption. When you send an encrypted message to a non-ProtonMail user, they receive a link which loads the encrypted message onto their browser, which they can decrypt using a passphrase that you have shared with them. You can also send unencrypted messages to Gmail, Yahoo, Outlook and others, just like regular email."