This is why you sign and encrypt the contents of email. If the recipient doesn't have the public key, they can't read the content.
Allowing a service provider to "handle your keys" is tantamount to letting the fox watch the henhouse.
Proton doesn't provide IMAP/SMTP access for free accounts, so you won't be able to encrypt emails locally.
This ultimately is the tech version of "trust me bro". This means you are as secure on Proton as you are on GMail, depending upon how you use the service.