https://www.openkeychain.org/ - github

OpenKeychain for Android helps you communicate more privately and securely, compatible with the OpenPGP standard.

K-9 Mail

https://k9mail.github.io/
https://github.com/k9mail/k-9 Github Repo Stars: k9mail/k-9

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