Please include:
1) The GNU Privacy Guard (GnuPG), "the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC4880. GnuPG allows to encrypt and sign your data and communication [...]"
2) Enigmail: "A simple interface for OpenPGP email security". "Enigmail is a security extension to Mozilla Thunderbird and Seamonkey. It integrates the renowned OpenPGP standard provided by GnuPG."