Mozilla the company behind the overnight success Firefox 1.0 has launched an email client to go up against Microsoft's Outlook Express email client.

The new open source application by Mozilla will feature a user-trainable junk mail filter and a built-in RSS reader alongside its core email elements. Like Firefox, the company is also promising a more secure package.

Downloads of early beta versions of Thunderbird have already surpassed 1million. The Web browser has been downloaded just over 9m times since November 9, according to the Spread Firefox website.

Thunderbird 1.0 initially is available only in English and is available for Windows, Linux, and Apple's Macintosh OS X operating system.

Thunderbird 1.0 is available as a free download from