Enigmail is a free add-on (extension) for the Thunderbird open source email client. Enigmail can encrypt and sign emails using GPG (GnuPG).
The EnigUsab project was initiated by Keith Andrews and Ralph Wozelka in 2015. It seeks to improve the usability of Enigmail, by finding usability problems through usability testing and then submitting fixes to the Enigmail project for (some of) those problems.
https://cdn.isds.tugraz.at/uxdaygraz2017/videos/andrews-uxdg-2017-11-11-showcase.mp4
http://keithandrews.com/talks/2016/glt-2016-enigusab/