Here we'll illustrate how to get Enigmail up and running.
As a first step, if you don't already use it, you need a Mozilla mailclient; the most used one is Thunderbird. For better readability, we will always refer to "Thunderbird" in our documentation, although technically correct would be "a Mozilla-based mailclient such as Thunderbird, SeaMonkey or Icedove". Instructions for Thunderbird should apply to all derivatives based on recent Mozilla code.
Second, you need the Enigmail extension. How to install these two will be covered in the next paragraphs.
As a third and fundamental requirement you need GnuPG which will be used to perform all cryptographic operations. If it is not installed on your system (or Enigmail cannot access it), the Setup Wizard will offer to download and install it for you.
Many Linux distributions ship with their own customized version of Thunderbird. If you use your distribution's version of Thunderbird, you should use your distribution's version of Enigmail. The package manager of your distribution will take care of the installation; you can then skip the rest of this paragraph and proceed directly to the Setup Wizard.
If you got Thunderbird from the Mozilla site and installed it yourself, though, you can use the official Enigmail releases provided by the Enigmail Project.
Note to advanced users: Enigmail has only been tested with official Mozilla releases of Thunderbird and SeaMonkey. If you use a nightly build (or your own build), Enigmail might not work.
Thunderbird has its excellent documentation and user group support, hence we'll skip the steps to install and configure it.
In any case you should have Thunderbird and your email account fully configured before proceeding to the installation of Enigmail.
Enigmail is a Mozilla extension. Extensions can be installed (and removed) through Thunderbird’s Add-ons Manager.
On Mac OS X, select Tools → Add-ons from the menu:
On Windows, open the menu and select Add-ons:
You can now select Enigmail from Extensions, or click on Search and search for it. Once you've found it, click on Install:
You will get a confirmation dialog warning you not to download software from unknown sources. However, the Mozilla site can be trusted and you can safely confirm. After installing the extension, you must restart Thunderbird to activate it.
Linux users: If you install all your software packages from a repository, you may find that it contains an older (but security-patched) Thunderbird version. Please use the (matching) Enigmail package from that repository.
If Enigmail is not shown in the Extensions list in when searching for it in the Add-ons Manager, then either Enigmail is already installed or your Thunderbird version may not be officially supported (i.e. available through the Enigmail add-on page). There are also current development snapshots available for test drive. Warning: Depending on the state of development, these builds may be unstable!
To install a downloaded XPI file, drag and drop it to the Extensions tab of Thunderbird’s Add-ons Manager or use its Install Add-on from file.