Enigmail is an e-mail plug-in for Mozilla Thunderbird and Seamonkey that allows you to send and receive secure, private e-mail. If normal e-mail is like writing a postcard, encrypting e-mail is like putting your mail in a sealed envelope.
To use Enigmail, you need to install three components.
To use Enigmail, you need to install version 1.4.9 or newer of the GNU Privacy Guard, also known as "GnuPG" or just "GPG".
For Windows users, there is an installer available on the GnuPG site. (Alternatively, Enigmail also works with GPG4WIN, which is similar to GnuPG.)
Mac users running Macintosh OS X 10.4 ("Tiger") or later, can use MacGPG. Those using Fink can just type fink install gnupg in a terminal window. Those using MacPorts (formerly known as "DarwinPorts") can just type sudo port install gnupg in a terminal window.
To use Enigmail with Thunderbird, you'll need at least Thunderbird 2.0.
Download the Enigmail plug-in by downloading and saving the .XPI file. (Do not let Firefox try to install it. That's not what you want.) Then start Thunderbird; select "Tools", then "Add-ons"; click the "Install" button; select the .XPI file you downloaded; and follow the on-screen instructions.
Additional information, more detailed instructions, and helpful documentation can be found in the Enigmail Quickstart Guide on enigmail.mozdev.org. Additional information about GnuPG can be found on www.gnupg.org.