Enigmail is a security add-on for Mozilla Thunderbird that allows you to use OpenPGP to encrypt and digitally sign your emails and decrypt and verify messages you receive.

Enigmail enables Mozilla Thunderbird and SeaMonkey to send and receive digitally signed and/or encrypted messages using the OpenPGP standard.

To provide the crypto-features, Enigmail requires GnuPG to be installed. The latest versions of GnuPG contain the latest security fixes, so it should be up to date. If your operating system does not supply GnuPG by itself, you may download GnuPG for Windows

Features:

Encrypt/sign mail when sending, decrypt/authenticate received mail

Support for inline-PGP (RFC 4880) and PGP/MIME (RFC 3156)

Per-Account based encryption and signing defaults

Per-Recipient rules for automated key selection, and enabling/disabling encryption and signing

OpenPGP key management interface.