GnuPG is a free implementation of OpenGPG which allows users to encode/decode emails, documents and stuff, based on private/public keys logic
NB: GnuPG is the same thing as GPG, which is a command line tool to use encrytion system
First install GPG with brew
brew install gpg
Once GPG is installed, you can continue with GPGTools freeware. This one will help you to manage keys more easily
You’ll find everything you need on this tutorial
Airmail doesn’t require to pay for GPG support plan for MAIL native Mac application
You just have to install a specific plugin