Email Encryption Tools
https://gpgtools.org gpgtools is a self contained Macintosh implementation of the RSA algorithm, it is not your only choice for the Mac, the GnuPG three program option for Windows and Linux, described below, is also available.
For both Windows and Linux I recommend GnuPG as your best choice. This is not self contained, you actually need three programs:
Mozilla Thunderbird (https://www.mozilla.org-US/thunderbird/)
The Windows instructions for integrating these three programs can be found at: https://ssd.eff.org/en/module/how-use-pgp-windows Most Debian based Linux distros already contain all three programs, you just need to use the appropriate installer (e.g. ‘apt-get’ for Ubuntu) to complete the installation. Otherwise you can download the installation files from their respective websites.