The Mozilla Foundation has released its new e-mail client, Thunderbird, aiming at users of Microsoft's Outlook Express and Qualcomm's Eudora.

The stand-alone open-source e-mail application has been in development since early 2003 and offers features such as a user-defined junk mail filter and a built-in RSS reader. Its release comes just a month after the Foundation launched version 1.0 of its browser, Firefox.

Mozilla is pushing Thunderbird's claimed tight security, which includes not allowing scripts to run by default and not automatically downloading images sent in an e-mail. It also uses the Mozilla rendering engine to display HTML e-mail messages, not Microsoft's IE engine, making it immune to IE-related bugs.

While downloads of early versions of Thunderbird have already surpassed one million, the Mozilla Foundation doesn't expect Thunderbird to take off the way Firefox did, a representative for the group said. The Web browser has been downloaded an estimated nine million times since 9 November.

Thunderbird has its roots in the Netscape Messenger 4.x client and the Mozilla mail and Usenet newsgroup client. Thunderbird's development team was led by two main engineers supported by volunteers, said Scott MacGregor, one of the engineering leads behind Thunderbird.

The Mozilla open-source project was started in early 1998 by Netscape, which was acquired later that year by AOL. Last year, the people behind Mozilla created a foundation, largely funded by a $2 million pledge from AOL, to build, support and promote Mozilla products.

Thunderbird 1.0 is initially available only in English. Versions in about a dozen other languages should be available in a week or two, according to MacGregor. It is available for Windows, Linux and Mac OS X.