Thunderbird

https://www.mozilla.org/en-US/thunderbird/ - Open Source Email Client

Security

• Manually configuring Thunderbird for secure login
• Digitally Signing and Encrypting Messages
• Encrypting email with Thunderbird

Others

• Mailvelope - OpenPGP encryption for Webmail.

Add-Ons

• Enigmail - OpenPGP message encryption and authentication.
• Identity Chooser - helps you to always use the correct identity when composing a new email.