ProtonMail is an end-to-end encrypted email provider based in Switzerland that aims to bring secure, encrypted services to the masses. Founded in 2013 by CERN researchers, it currently has around 20 million users.
Web Key Directory (WKD) allows hosting of public PGP keys without maintinaing a Web Key Server (WKS). This way, compatible services can automatically fetch a recipient's public PGP key and seamlessly encrypt messages without needing to ask the recipient for their key beforehand.