Thunderbird comes with a built in addressbook and you can also install calendar to make it a complete personal information manager. Thunderbird serves as a news reader too. This allows you to subscribe to usenet newsgroups.
Unlike some of the available email clients, thunderbird doesn't allow scripts to be run by default. This makes it more secure against the internet worms. It also offers a very high level security needed at the enterprise level with features lie S/MIME, digital signing, message encryption, support for certificates and security devices.
One of the best features of thunderbird is junk mail control. It detects junk mail automatically based on certain criteria. You can also set it to move junk mail to specific folder as it arrives. There is definitely a learning curve for the junk mail filter and you can fine tune this with the help of Junk/Not Junk button. Junk mail filter is turned off by default. It can be turned on by clicking "Enable adaptive junk mail detection" in Adaptive filter tab of Junk mail Control window. This is one of the best in built junk mail feature I have encountered.
You can customize thunderbird to suit your personality. You can select one of the three available message views (Classic, Wide view, Vertical view) depending on your liking. You can also customize the toolbar in whatever way you want. In addition to these flexibilities, you can add any theme you like using options menu. You can either download themes from this site or you can create your own theme.
Another feature which you might find very convenient is spell checker. It comes very handy when you have to be careful with spelling like job application, business email etc.
Apart from the above mentioned features, the basic email client is packed with features like IMAP/POP support, support for HTML mail, labels, quick search, a smart address book, return receipts, advanced message filtering, LDAP address completion, import tools, powerful search, and the ability to manage multiple e-mail and newsgroup accounts. These features makes thunderbird stand apart from all the other email clients.
If you are not satisfied with the features described so far, thunderbird gives you an option to add more features using extensions. You can download extensions from Texturizer.net or Mozdev site. You can also create your own extension and add. These extensions add tons of more features to that already available. One such cool extension is weather which will show your local (or any place you select) weather condition. You can download weather (which is still in beta but I have no problems with it) from this site.
To sum up, this is the best email client I have ever used. I can even challenge readers to come up with another stable lightweight client with so many features on it. The best thing about this wonderful software is that it is free (as in free speech and free beer).