Open Source Secure Clients

ThunderbirdThunderbird Email Client

https://www.mozilla.org/en-US/thunderbird/

Enigmail OpenPGP encryption

Enigmail adds OpenPGP message encryption and authentication to your email client. It features automatic encryption, decryption and integrated key management functionality. Enigmail is based on GnuPG  for the cryptographic functions.  GnuPG is not part of the installation, but will be downloaded as part of the setup process.

https://addons.mozilla.org/en-US/thunderbird/addon/enigmail/?src=cb-dl-featured

Lightning Calendar for Thunderbird

Lightning allows you to organize your schedule, access remote calendars and much more.

https://addons.mozilla.org/en-US/thunderbird/addon/lightning/?src=cb-dl-featured

Gpg4Win

Pretty Good Privacy for Windows for file and email encryption. Gpg4win (GNU Privacy Guard for Windows) is Free Software and can be installed with just a few mouse clicks.

https://www.gpg4win.org/

GPGtools MAC Pretty Good Privacy

Use GPG Suite to encrypt, decrypt, sign and verify files or messages. Manage your GPG Keychain with a few simple clicks and experience the full power of GPG easier than ever before.

https://gpgtools.org/

OpenKeychain PGP for Android

https://www.openkeychain.org/

GNUPG Linux / Unix PGP

https://www.gnupg.org/

K-9 Mail Android Secure Email Client

https://securityinabox.org/en/guide/k9/android

iPGMail iphone PGP app costs $1.99

https://ipgmail.com/