ProtonMail is a secure encrypted email provider, which runs a “zero access” PGP mail service based in Switzerland. (Read More)
Since, ProtonMail doesn’t allow direct signup and they only accept the user through the invitation.
I had also signed up for ProtonMail to see the UI and all the things & go their invitation. I checked for some vulnerabilities and couldn’t find anything from the inside. Then I suddenly went through the invitation link which I was registered.