https://autocrypt.org/

Autocrypt is a set of guidelines for convenient end-to-end-encryption of e-mails. Developers are working on bringing it to several different e-mail programs.

https://autocrypt.org/level1.html autocrypt-spec-1.0.0.pdf

Delta Chat

https://delta.chat/

Delta Chat is a modern messenger. It is like email in a new dress. Just better, safer and user-optimised.

Enigmail

https://www.enigmail.net

Enigmail is a seamlessly integrated security add-on for Mozilla Thunderbird. It allows you to use OpenPGP to encrypt and digitally sign your emails and to decrypt and verify messages you receive.

K-9 Mail

https://k9mail.app/
https://github.com/k9mail 
https://github.com/k9mail/k9mail.app 

K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email

muacrypt

https://muacrypt.readthedocs.io/index.html

muacrypt provides a command line tool and a Python API to help mail agents, both user and server-side, integrate and manage automated e-mail end-to-end encryption with https://autocrypt.org.

PyAC

https://pyac.readthedocs.io/en/latest/

Autocrypt Level 1 implemention using PGPy . Initially part of py-autocrypt, which uses GnuPG.