Tutanota is a free and open source web-mail client that focuses on privacy. It currently has an Android and iOS app, and offers easy end-to-end encryption.
Tutanota provides end-to-end encryption, so email stored on Tutanota’s servers is encrypted and cannot be accessed or decrypted by staff members. It also permits users to signup anonymously over Tor.
Emails between Tutanota users are encrypted using ‘a standardized, hybrid method consisting of a symmetrical and an asymmetrical algorithm,’ using 128-bit AES with 2048-bit RSA handshake encryption, while emails to non-Tutanota users are encrypted using AES-128. Finally, this is private, ad-free email, where your email data can not be analyzed for ad display purposes.
Proprietary applications it could replace: