Info on Encryption – use GnuPGP or PGP
Using GnuPG (Windows or Linux):
I use GPG, from GnuPG, the complete and free implementation of the OpenPGP standard. It’s convenient, has better cross platform compatibility and is open source! – but you could use PGP
For Linux, there are a greater variety of options. GPG is conveniently available in most distros.
Plugins for many other mail programs are available off the GnuPG pages, under “Frontends“.
For more information and links, check: