Proton Mail (Web, Android, iOS) is an open-source email service that uses end-to-end encryption and zero-access encryption to protect the user’s data and privacy.
Riseup Email is a secure email service by activists for activists. Its security features include full-disk encryption, encrypted email storage, Onion routing, and more.
Skiff Mail is end-to-end encrypted email that protects your inbox and gives you the power to communicate freely.
Tutanota provides secure, green, and ad-free email service that incorporates privacy by design.