The Thunderbird 2 e-mail program may not get as much love from its makers at the open-source Mozilla Project as their favored child, the Firefox Web browser, does. Nevertheless, Thunderbird 18.104.22.168 has a very strong feature set, and does its job well; plus, if you are looking for an e-mail client that works and looks exactly the same on most of the different operating systems you may have to deal with, then Thunderbird's almost the only game in town. I suspect most Mac OS X users will prefer Apple's Mail, if only because it's prettier; unlike Thunderbird, Mail doesn't have to make aesthetic compromises for the sake of compatibility with Windows and Linux. But if you care more about features than looks, Thunderbird is definitely worth a free download.
Managing your mail
Thunderbird helps you manage your mail in a number of ways, some of them fairly clever. For starters, Thunderbird 2 has very good built-in spam filtering that keeps most of the junk mail out of your inbox. And if very good isn't good enough for you, you can easily move up to best in class with C-Command Software's SpamSieve, which supports Thunderbird 2. Once mail is downloaded to your computer, Thunderbird makes it easy to search through it; it's also possible to save searches as special folders that are very similar to Apple Mail's smart mailboxes.
Thunderbird 2 offers a couple of distinctive user-interface features that I like a lot. The folders pane in Thunderbird 2 offers a variety of ways of looking at what you've got. One view shows only your favorite folders; you can of course add and remove folders from the list of favorites. Another view shows only folders containing unread messages; and there are others. This may be Thunderbird 2's most distinctive feature, and if you like to experiment with folders, this is extremely useful.
Another new feature in Thunderbird 2, and one I haven't seen in other e-mail programs, is the viewing history. Thunderbird provides Back and Forward buttons that let you jump back to a message you were reading or composing a few minutes ago, then jump forward to something else, just as you can move forwards and backwards in the page viewing history of your Web browser. I have not found this to be terribly useful myself, but some people might. Thunderbird also has a threaded view, but it is not as flexible as the threaded view in Mail, and it frequently fails to group messages together correctly if they lack metadata that identifies them as belonging to the same thread.
You can create as many labels as you like in Thunderbird, and any message can have several tags applied. This allows you to organize and find messages in several ways.
What I like best about Thunderbird 2 is something that it shares with my all-time favorite e-mail client, Google Gmail: complete support for tags. (Gmail calls them labels, but they work the same way.) In Thunderbird, you can create as many tags as you want or need, which is important; but the really great thing about tags is that you can apply more than one to the same message. Why is this useful? Because a lot of messages fall into more than one organizational category. For example, if a relative writes to ask me to photograph their wedding, I can tag the message both as Family and Photography. If I had only folders to organize my messages, I'd have to decide whether to store the message in the Family folder or the Photography folder--and sooner or later, I wouldn't be able find it when I wanted to, because I'd be looking for it in the wrong place.
When tags are properly and fully implemented, as they are in Thunderbird, they provide a way of organizing messages that is more flexible than the use of folders because it's multidimensional. Even smart folders are less useful, because messages are found by smart folders only if they meet certain specific criteria, while tags can be applied both by filters (so they can be specific and automatic) and manually (so you can catch all the exceptions).