Thunderbird is a full app, but not yet a full suite. You can add instant messaging (chat, at least) with the Chatzilla Extension; you can add calendaring with the Calendar Extension. What's still a few months away is Lightning, which integrates calendar alerts with Thunderbird e-mail. Both Evolution and Outlook do that today. When Lightning's finished, Thunderbird will be a full-on Outlook competitor.
Thunderbird is great as it is for home e-mail, for point-of-presence e-mail, and for situations where calendaring and groupware are lower priorities than security. Thunderbird's e-mail security is very high quality compared to (say) Outlook Express.
Thunderbird is "just" a client, but since it runs the same on Unix and Linux as it does on Windows, you can run it on servers as well. In fact, some people with fast local networks, like Gigabit Ethernet, recommend running VNC on the local computer and accessing a Linux desktop and Thunderbird on the server through that. Then you get to throw Windows away on all the clients, and run multiple desktops all at once on your fast server hardware.
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