AnonAddy
AnonAddy provides an email forwarding service that keeps personal email addresses private.
K-9 Mail
K-9 Mail is an open source email client for Android with OpenPGP encryption support.
Mailvelope
Mailvelope is an open-source extension for Chromium- and Firefox-based web browsers that provides an interface for Pretty Good Protection (PGP) email encryption on popular web-based email services.
Proton Mail
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
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.
Thunderbird
Thunderbird is an open-source desktop email client that has built-in security and safety features including remote image linking protection, phishing protection, PGP encryption, and more.