ProtonMail is the worlds largest encrypted email provider with over 2 million users worldwide. The service is a free and open source, featuring strong end-to-end encryption and protected by Swiss privacy laws. It uses a digital certificate issued by Digicert, the same CA used by Facebook for its Tor hidden service. The company used spare CPU capacity to generate millions of encryption keys and then hashed them aiming to generate a more human readable hash. The address it can be easily remembered as: Protonirockerxow.onion.”]
Source: http://securityaffairs.co/wordpress/55471/digital-id/protonmail-tor-hidden-service.html