Electronic Frontier Foundation
To use PGP to exchange secure emails you have to bring together three programs: GnuPG, Mozilla Thunderbird and Enigmail. GnuPG is the program that actually encrypts and decrypts the content of your mail, Mozilla Thunderbird is an email client that allows you to read and write emails without using a browser, and Enigmail is an add-on to Mozilla Thunderbird that ties it all together.
What this guide teaches is how to use PGP with Mozilla Thunderbird, an email client program that performs a similar function to Outlook. You may have your own favorite email software program (or use a web mail service like Gmail or Outlook.com). This guide won’t tell you how to use PGP with these programs. You can choose either to install Thunderbird and experiment with PGP with a new email client, or you can investigate other solutions to use PGP with your customary software. We have still not found a satisfactory solution for these other programs.