On the heels of the success of its open-source Firefox browser, the Mozilla Foundation on Tuesday will release an E-mail client intended to be an alternative to Microsoft's Outlook Express.

Thunderbird 1.0, available as a free download at www.mozilla.org, is designed to work with Firefox 1.0, the browser released in November that's cutting into the market share of Microsoft's Internet Explorer. Mozilla developers are working on a related calendaring application for release in 2005.

Thunderbird runs on Windows, Linux, and Mac computers. Lead engineer Scott MacGregor says Thunderbird come with built-in defenses for two of the most common E-mail problems: spam and viruses. Other features includes an RSS viewer for reading and managing Weblog postings, a Saved Search function that makes it possible to save searches within a folder, and the ability to group E-mails by day or week.

Thunderbird 1.0 is aimed at consumers, but MacGregor expects it to appeal to business users, too. Already, one company has deployed 40,000 copies of Thunderbird, MacGregor says, though he declined to name the company. A tool called Mission Control Desktop allows system administrators to centrally manage Thunderbird functions such as user settings.