After looking at the doc, it seems Protonmail uses GPG and computes the keys on the client device. (So I was wrong in my reply earlier). However, when you send

After looking at the doc, it seems Protonmail uses GPG and computes the keys on the client device. (So I was wrong in my reply earlier). However, when you send an unencrypted email with Protonmail, they claim that they encrypt the email at rest, but the claim is not verifiable.