Secure encrypted e-mail is possible using gnupg – the Open Source alternative to pgp – for details of gnupg see http://www.gnupg.org/
This allows keys of 2048 (and above) bits, which are unbreakable even by Imperialist (Amrika!) intelligence agencies. Note that the current 1024 bit gnupg key is also regarded as unbreakable at the moment.
In addition, several open source MUA’s (Mail User Agents) are available which allow for easy sending and receiving of encrypted e-mail. Among the best and easiest to use is mutt – see http://www.mutt.org/
Most major distributions of the Linux operating system include both gnupg and mutt. Debian GNU/Linux – see http:/www.debian.org – is probably the best Linux operating system available.