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 didnt 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 Googles 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 PCMag.com and our sister sites eWEEK.com and Microsoft-Watch.com. The mechanism for adding feeds is rudimentarythere 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 globalyou cant, for example, use the standard view for e-mail and the wide view for RSS feeds.