I have been using Mozilla Thunderbird for several years now. There are some reasons it has become my favorite email client. Firstly, I sometimes switch between Linux and Windows, and for the multi-platform usage Thunderbird is ideal, since you do not want to use two different clients. Secondly, this application not only manages your emails but also does many other things. And moreover, its features can be expanded by using any from a long list of available add-ons. In addition, if the application's interface seems a little plain to you, it is possible to change its look by applying any of the available skins.
If you are already familiar with Microsoft Outlook, starting to use Thunderbird will surely be an easy task. Moreover, you can use the tabs in the interface to display messages or access other services. Obviously, you need to set an email account before you can start managing your mail inbox. Mozilla Thunderbird will make this task very easy and even offer to create a new account for you. Fortunately, you can set various email accounts, which you can compartmentalize by using different folders. Not only mail accounts can be managed but also chat accounts and RSS. Fortunately, it supports multiple chat servers, such as XMPP (Jabber, Facebook), Google Talk, IRC, Twitter and Yahoo. Finally, the built-in calendar (Lightning) allows managing your events and triggering alerts, what is more, your calendar can be stored locally or on a server.
The program uses a fast rendering engine, so you will have almost instant access to the contents of your messages. And talking about contents: this program can make dealing with spam easier, as its developers have implement Bayesian algorithms that allow it to actually learn from your decisions.
All in all, Mozilla Thunderbird is undoubtedly one of the best email clients. It is constantly being developed by a legion of devoted followers; and fortunately, the program can connect to the Internet and check your version so that you are always up-to-date. Perhaps, my only objection is that I find that chatting is somewhat slow compared to other messengers.