What is GPG?
GPG stands for GNU Privacy Guard (http://www.gnupg.org ) and is a complete and free replacement for PGP (Pretty Good Privacy). GPG is compatible to the standard OpenPGP (RFC 2440).
In a company there exists lots of confidential information, that is unfortunately sent by mail very often. This information may be offers, employee data or the latest source code of your brand new product. I think nobody wants to make it possible that a competitor is reading your latest offer or your source code. As with privacy of correspondence in conventional letters, protecting your correspondence should become naturally also with electronic mail. Especially when encrypting with modern programs is done with a single mouse click.
Keep in mind, an email is like a postcard. Everbody can read it and when sending it through the internet it goes through many servers before it reaches the recipient.
Did you ever get a virus mail or spam with your sender address? You can write any mail address in the From: part of a mail. You can't prevent that, but you can debilitate such mails by consequentially signing your mails. It's almost impossible to fake such a signature. And when receiving a mail which is signed with one of your trusted keys you can be sure that the sender is who he is pretending to be.
E-Mail-Client: Mozilla Thunderbird [Screenshot 1] [Screenshot 2]
GPG-Plugin: Support and download is available from www.enigmail.net
GPG for Windows: GPG4Win Includes gpg, a key manager, as well as Plugins for Outlook and Explorer
For the Outlook family there are several commerical products available.
Tip: You should configure these products OpenPGP compatible, because proprietary extensions may lead to problems.
All popular distributions already contain GPG.
E-Mail-Client: KMail has excellent GPG support.
Thunderbird is also available for Linux.
Tip: KGPG is a pretty good tool with an intuative user interface for managing GPG keys.