Enigmail is an extension to the mail client of Mozilla / Netscape and
Thunderbird which allows users to access the authentication and
encryption features provided by the popular GnuPG software (see
screenshots). Enigmail is open source and dually-licensed under the GNU
General Public License and the Mozilla Public License.
Having tried e-mail encryption of many flavors over the years, this
sounds like it might just work. Integration has always been the killer.
Either the system used a standard encryption tool such as PGP or now
GPG, or it used a built-in system that didn't necessarily play nicely
Enigmail seems to solve both of those problems. [Jeremy Zawodny's blog]