Use GnuPG, an implementation of OpenGPG standard. Some emails clients support natively, others needs a plugin or external program. The OpenPGP site has a list, but misses some, see the g10code.de site. Most important,
- Thunderbird: use Enigmail plug-in
- Outlook Express: use GPGOE extension, or WinPT’s OE extension
- Outlook: GPGOL, supposed replacement for gdata (gdata mentioned a lot in old email but last News dates from 2002!). See also this post by nullamatrix.
- any: straight gpg on Linux (must use console), and WinPT on MS Windows (en/decrypt/sign/verify using clipboard, so universal). Unfortunately WinPT last update is from 2003! But it seems to be much in use, e.g. it is part of GPG4win, the windows package for GPG.
Programs called “relays” are also possible, they intercept all smtp communication and secure stuff on the fly, e.g. see http://www.bretschneidernet.de/tips/secmua.html.
This site has many links to plugins tabled according to platform etc, as well as how-to’s etc: http://www.vanheusden.com/pgp.php
