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

Writing

• Switch Between HTML And Plain Text Emails In Thunderbird

Web Access

Access your IMAP email account through a web client:

• roundcube - open source webmail software.
• rc_openpgpjs - extension adding OpenPGPs functionality to the Roundcube webmail project
• Mailvelope - OpenPGP encryption for Webmail.
• OpenPGP.js - Open Source OpenPGP library in JavaScript so it can be used on virtually every device.
  • Simple HTTP form

Other Security Topics

• The Best Free Ways to Send Encrypted Email and Secure Messages

Add-Ons

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