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Based on our initial impressions, the new open-source e-mail client from Mozilla, called Thunderbird, is well worth exploring. It seems to work quite well, both on Web-based mail like Gmail and on POP3 accounts. It also supports IMAP for connecting to Exchange and other servers, but we didn’t test that feature.

Outlook Express users should have no problem understanding and using Thunderbird. It works in much the same way as Outlook, with buttons on the top to get mail, a preview pane to read messages quickly, and an easy way to compose and respond to e-mail.

Setup was easy. We simply downloaded the application from the Mozilla site. The program walked us through setting up a connection to a POP account, and we also quickly connected Thunderbird to Gmail using instructions on Google’s site.

Thunderbird adds a number of new features that could make it superior to Outlook Express. Our favorite is the ability to integrate RSS feeds into the e-mail client. During testing, we added feeds from and our sister sites and The mechanism for adding feeds is rudimentary—there are no predefined feeds, and the e-mail client will not auto-discover feeds from a Web page. Once you subscribe, however, Thunderbird makes it simple to view those feeds.

The feed display console is identical to the e-mail display. Feed entry headlines are displayed on top, and the preview window displays either the entire Web page or a summary of the feed. This makes it easy to browse through multiple feeds and keep up with blogs and other news services.

You can also select from one of three different preview panes. The default view puts your folders on the left side, message list on top, and preview underneath. A wide view extends the preview window to the full width of the screen, while the vertical view puts the folders, message list, and preview into separate vertical columns. Unfortunately, view settings are global—you can’t, for example, use the standard view for e-mail and the wide view for RSS feeds.

Thunderbird also surpasses Outlook Express with its e-mail creation capabilities, at least on preliminary tests. It supports a wide range of HTML-style e-mail constructs, including tables, named anchors, and even a table of contents. It also provides more powerful image tools than those found in Outlook Express. Images can be easily resized and placed anywhere in the message. Like Outlook Express, Thunderbird includes basic text formatting and a spell-check feature.

The Mozilla-bred e-mail client also includes what appears to be more advanced inbound-message processing. It includes a junk e-mail filter that can automatically flag messages it thinks are spam, with the option of automatically moving or deleting them. It didn’t work very well at first, but the program can also be taught what’s junk and what isn’t. Thunderbird also includes a basic rule-processing capability—we found it equivalent to what’s in Outlook Express, though with a somewhat different interface.

Mozilla has also made Thunderbird natively extensible—both in terms of functionality and look and feel. Although only nine Thunderbird extensions are available today, more are likely to be released over time. The current list of extensions includes the ability to control your music player from within Thunderbird and a nifty way to use mouse gestures, instead of the keyboard, to read e-mail. Thunderbird’s look can also be changed through downloadable skins. We couldn’t test this feature, however, because the skins available on Mozilla’s Web site haven’t been updated from the beta, and therefore would not work with Version 1.0.

All in all, Thunderbird seems like a better e-mail client than Outlook Express. If you use POP or Web e-mail, give it a try. Chances are, you won’t go back.