Uhm you can curl https://api.protonmail.ch/pks/lookup?op=get&search=$email_address https://api.protonmail.ch/pks/lookup?op=get&search=$email_ad... for any valid

Uhm you can curl https://api.protonmail.ch/pks/lookup?op=get&search=$email_address https://api.protonmail.ch/pks/lookup?op=get&search=$email_ad... for any valid $email_address and get the public key. I have used this to send signed/encrypted mail to a ProtonMail recipient. It worked, until he responded inline without encrypting it to my private key, thereby completely defeating the point. (Later I informed him of how to automatically sign and encrypt outgoing mails to my account, as that is possible too, but not obvious at all.) PM should make the more obvious, but in principle the interoperability is there and works.