I've been using GnuPG in combination with GPGshell for several years. GnuPG provides a command line encryption engine and GPGshell provides a GUI interface. Public key encryption requires a fair bit of set up and training for non-technical users but nothing is more secure. Since GnuPG is open source you KNOW there are no back doors. GnuPG is compatible with all but very old PGP encryption algorithms and is free for everyone to use. PGP is free for personal use only.
A Mozilla Thunderbird extension know as Enigmail (http://enigmail.mozdev.org) integrates GnuPG, allowing easy digital signing and encryption directly from within the application. While the setup still uses the command line functionality, once set up, sending and recieving secure messages is a breeze.
The trio software of "gnupg + Enigmail + Thunderbird" is wonderful for encrypted emails. My six year old son and I have become secret agents now and we keep his older brothers out of his emails. It's better than the dial-up decoder discs I made! Yee-haw!
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