What is Mozilla Thunderbird?
Mozilla Thunderbird is an e-mail and newsgroup client.
What license is it under?
Thunderbird is open source and available under the Mozilla Public License. The MPL is a free software license that is not strongly copyleft.
What does it run on?
Windows, Linux, Mac OS X, Solaris, Java Desktop System, OS/2.
Why Use Thunderbird?
It is Free software that uses Open Standards. This means that not only can you can use it on machines that run more than one operating system, you can be sure that your data is always available and usable, whatever software environment you choose. Thunderbird itself is cross-platform, so you can use it on most common machines.
And it's user-oriented, with a great interface.