Note that ProtonMail actually supports automatic encryption to email accounts that publish their public keys in a Web Key Directory, which I’ve set up for mine. When you type such an email address in the To field, it’ll turn into a special color with a lock symbol.

Likewise, ProtonMail also exposed a WKD so people can send encrypted emails to ProtonMail accounts. I don’t know of any mail clients that support this though (I used the command line to pull keys)