OpenGPGMail

OpenGPGMail is a plugin for OS X's Mail.app, which let's you
send and receive secure, OpenPGP encrypted and signed messages.

OpenGPGMail is a fork of GPGMail from GPGTools. I decided to embark on this project when a relatively quiet update to the software introduced a banner in Mail mandating that purchase a support plan, or else.

I feel the viability of PGP in general depends on widespead adoption, and introducing a barrier to otherwise intuitive software works counter to that idea.

The remaining components of the GPGTools software suite remain truly free, for now. I'd recommend installing those tools directly from their official website.

Localizations are done on Transifex.

Prerequisite

In order to use OpenGPGMail you need to have GnuPG installed. You can either

build your own version
use one from homebrew
find a packaged version for OS X at gpgtools.org
Libmacgpg (https://github.com/GPGTools/Libmacgpg.git) To build this software, you need to have the full version of Xcode installed.

Build

Clone and make

git clone https://github.com/jbharter/OpenGPGMail.git
cd OpenGPGMail
make

Install

Copy GPGMail.mailbundle to the user library mail bundles

killall Mail
rm -rf ~/Library/Mail/Bundles/GPGMail.mailbundle
cp build/Release/GPGMail.mailbundle ~/Library/Mail/Bundles/GPGMail.mailbundle

System Requirements

Mac OS X >= 10.9 (macOS Sierra is not yet supported!)
Libmacgpg
GnuPG