Enigmail is an excellent and classic add-on! Rated 5 out of 5 stars
Don’t be put off by the review below! With a willingness to learn a little about encryption and secure and trusted communication, and a bit of common sense, you can set up encrypted e-mail on Thunderbird practically by selecting the defaults.
As pointed out in the details above, you will need GnuPG, so the first thing to do is go to http://www.gpg4win.org/ and download GPG4Win (this is for Windows, obviously). After installing it, you will find a folder on your desktop containing the GPG4Win Compendium PDF. This contains an excellent simple explanation of the principles behind secure and trusted communication, and why you should care about it. I have put the PDF on my tablet and read it repeatedly. You have to know a little about the fundamentals before you can understand what the e-mail setup is actually about.
Now start Thunderbird, and you should see an OpenPGP menu item. There is a setup wizard to get you started. I just selected defaults where I wasn’t sure, and had the whole thing set up in less than half-an-hour. Nothing to do with the command line. It was simpler than setting up Outlook to do the same. But you have to know what’s going on, so read the Compendium first.
If all you want is to send someone an encrypted message once a month, I recommend the Simple Text Encryptor tool from here: http://sourceforge.net/projects/simpletextenc/; you just copy and paste your message into the top box and it comes out encrypted to AES 128-bit standard at the bottom – not quite as simple as pressing “E” but not far off!
You don’t need to attend a seminar to use this tool, but you do need to be prepared to learn something new, and to do a little more than press E.
We may not take it particularly seriously at the moment, but there are countries where tools like this are an absolute necessity to maintain the human right of privacy, so it’s great that they are available free. And the way the UK and US governments are heading, we may need such tools sooner than we think. Thank you, Patrick Brunschwig and the GnuPG team!
Finally, as a REVIEWER, you NEED to be AWARE that if you type INANE reviews with LOTS of CAPITALS, and “inappropriate use” of quotation marks, and odd *placement* of *star* symbols, it’s going to REALLY HACK some people OFF, as well as GIVING them SERIOUS DOUBTS about your LEVEL of MATURITY. Verb. Sap.