Enigmail

Enigmail is a data encryption and decryption extension for Mozilla Thunderbird and the Postbox that provides OpenPGP public key e-mail encryption and signing. Enigmail works under Microsoft Windows, Unix-like, and Mac OS X operating systems. Enigmail can operate with other mail clients compatible with PGP/MIME and inline PGP such as: Microsoft Outlook with Gpg4win package installed, Gnome Evolution, KMail, Claws Mail, Gnus, Mutt. Its cryptographic functionality is handled by GNU Privacy Guard.

In their default configuration, Thunderbird and SeaMonkey provide e-mail encryption and signing using S/MIME, which relies on X.509 keys provided by a centralised certificate authority. Enigmail adds an alternative mechanism where cooperating users can instead use keys provided by a web of trust, which relies on multiple users to endorse the authenticity of the sender's and recipient's credentials. In principle this enhances security, since it does not rely on a centralised entity which might be compromised by security failures or engage in malpractice due to commercial interests or pressure from the jurisdiction in which it resides.