proton.me
Proton Mail is a secure email service operated by Proton, based in Switzerland and protected by Swiss privacy law. It is the world's largest secure email service, with over 100 million users, and offers end-to-end and zero-access encryption for all messages. The service is ad-free, available on web, iOS, Android, and desktop platforms, supports importing existing messages and contacts, and offers both personal and business plan options.