This post forms part of the series on Securing Leopard, and covers GPGMail, Mail.app plugin that allows you to digitally sign, encrypt and decrypt emails using PGP/GPG
"Select your email program
Select your email program from the list and click the button at the bottom to display the configuration instructions for your program. "
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
"OpenPGP works using GnuPG - you will need to download this to use OpenPGP email encryption and signing. GnuPG is a command-line program that can be difficult for inexperienced users to grasp. Fortunately, there are many GnuPG "frontend" programs that add a graphical user interface to it, allowing you to easily incorporate features of OpenPGP to your communications."