Finally, while Thunderbird has a good Bayesian spam filter, it doesfor reasons I cant tell, and no one else can as far as I can tellsometimes mark messages as spam, but then it doesnt automatically trash them as Ive set it up to do. Its no deal breaker, but it is annoying. After all that, you may wonder why I say I like Thunderbird. Well, first there are all the reasons we list in our latest review of Thunderbird. In addition to the ones listed there, I also favor Thunderbird because its more secure than Outlook or Outlook Express. Both Microsoft mail programs have come a long way since I called Outlook a security hole that pretended to be an e-mail client, but Thunderbird starts out more secure than either one since it doesnt have roots into the actual operating system.Besides, while Internet Explorer vulnerabilities get most of the press, Outlook still has major holes in it. For example, according to SecurityFocus, Outlook 2003 still has a media file script execution hole that could be used to run programs on your PC.All in all, if Im running e-mail on Windows, Id rather have Thunderbird, warts and all, rather than an eternally insecure Microsoft product. But thats me. For businesses, especially large ones, Thunderbird really, really needs group calendaring and schedule management capabilities. The Mozilla folks are working on giving us just such capabilities in the Mozilla Calendar extension project, but, in the current 0.2 release, its still a long ways off. In addition, for group calendaring, the Calendar relies on WebDAV (Web-based Distributed Authoring and Versioning), and its not clear to me how popular WebDAV is going to end up being. Id be happier if it supported WebDAV and Microsofts Exchanges MAPI (Messaging API). Like it or lump it, Exchange and MAPI are well-entrenched in many companies groupware infrastructure. Now you can get an open-source e-mail client that supports MAPI scheduling and, thus, Exchange group calendar. But its not Thunderbird, its Novells Evolution and Exchange Connector. I like Evolution, but it runs only on Linux and Solaris. For Windows users, Thunderbird is clearly the open-source e-mail client of choice. Still while I can recommend Thunderbird for computer-savvy home users and small businesses in a heartbeat, I cant recommend it for naïve users or big businesses. That said, given the incredibly rapid rate at which the Mozilla Foundation improves its programs, I have little doubt that by this time next year, Thunderbird will be at least as popular as Firefox is now. eWEEK.com Senior Editor Steven J. Vaughan-Nichols has been using and writing about operating systems since the late 80s and thinks he may just have learned something about them along the way. Check out eWEEK.coms for the latest open-source news, reviews and analysis.