ProtonMail is an end-to-end encrypted email, it uses client-side encryption to protect email contents and user data before they are sent to ProtonMail servers, in contrast to other common email providers such as Gmail and Hotmail. That means you cannot decrypt during transfer, even ISP and protonmail themselves cannot decrypt (at least at prensent). Their datacentre located at Switzerland, so they will not effected by US government like Lavabit.
Although protonmail is still not a fully secured way, but it is a choose if you don’t trust google or microsoft.
reference link :