OpenPGP & Enigmail
http://enigmail.mozdev.org/home/index.php.html

Quick start guide
http://enigmail.mozdev.org/documentation/quickstart.php.html
http://enigmail.mozdev.org/documentation/enigmail-quickstart.pdf

Install GnuPG on local computer
http://www.gnupg.org/

Windows uses version 1.4.11 as of 10 February 2011 - accept default settings
ftp://ftp.gnupg.org/gcrypt/binary/gnupg-w32cli-1.4.11.exe

Install right Enigmail package for your system.
http://enigmail.mozdev.org/download/index.php.html
Download the .xpi file to desktop and then use add-ons in Thunderbird and Seamonkey to install the package from the locally-saved file.

After installing, open the OpenPGP Preferences and make sure the GnuPG path is set (/usr/bin/gpg or C:\Program Files (x86)\GNU\GnuPG\gpg.exe) and restart Thunderbird.

Inside Enigmail, import local copy of pubring.gpg and secring.pgp
(Delete any old keys there)

Remember the key passphrase is stored securely elsewhere.

Import other people’s keys from http://pgp.mit.edu/

Test the sending and receiving of encrypted messages.

May want to adjust preferences inside Enigmail.

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Useful reference for using keys on Linux:

http://www.debuntu.org/how-to-import-export-gpg-key-pair

http://aplawrence.com/Basics/gpg.html

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If Enigmail seems to freeze, open an xterm and follow instructions below:

If you get this:

[juckins@metroliner: ~]$ gpg-agent
gpg-agent[18835]: can't connect to `/home/juckins/.gnupg/S.gpg-agent': No such file or directory
gpg-agent: no gpg-agent running in this session

Then follow tip on http://forums.gentoo.org/viewtopic-t-642131-highlight-gpgagent.html

juckins@metroliner: ~/.gnupg]$ eval 'gpg-agent --daemon --use-standard-socket'
GPG_AGENT_INFO=/home/juckins/.gnupg/S.gpg-agent:18933:1; export GPG_AGENT_INFO;

[juckins@metroliner: ~/.gnupg]$ gpg-agent
gpg-agent: gpg-agent running and available

If that all works properly, then edit .bash_profile like this:

# start gpg-agent (for Thunderbird Enigmail)
eval 'gpg-agent --daemon --use-standard-socket 2>/dev/null'

Other possible help: http://www.linuxquestions.org/questions/linux-security-4/gpg-gpg-agent-can%27t-connect-to-root-gnupg-s-gpg-agent-611843/