Mozilla has posted release candidate code for the newest version of its email client, Thunderbird 2.0, which emphasises message organisation and support for Windows Vista (see our first look at Thunderbird 2.0 here).

Unless the 50,000 testers in Mozilla's beta community find flaws with the Thunderbird 2.0 Release Candidate 1 code, the final version will be rolled out by the end of this month, said Scott MacGregor, lead engineer for Thunderbird.

Based on earlier Mozilla product road maps, Thunderbird is almost a year late. A December 2005 document laid out the then-current schedule for the email application and tagged the Version 2.0 release for summer 2006.

Among the biggest feature changes in RC1 from earlier previews, said MacGregor, is message tagging. "Users are used to tags on the web, like tagging images on Flickr," he said. "Users can create any number of tags, assign them to messages and then combine them with search folders to make it easier to organise email. It was clear after Thunderbird 1.5 that people were most interested in [the addition of] tags."

Tags, dubbed ‘labels’ in Google's Gmail service and ‘categories’ in Microsoft's Outlook email client, let users classify mail, sort it by those labels and pull up tagged messages in saved searches, which are generally presented in the usual folder metaphor of email.

Thunderbird 2.0 also features improved phishing detection by virtue of a more effective antiphish algorithm. The client doesn't yet use Firefox's downloaded blacklist of known phishing URLs, however; that will probably make it into the Version 3.0 release, MacGregor added.

Also new to Thunderbird 2.0 RC1 is integrated support for two webmail services, Gmail and Apple's .Mac; improved new mail alerts; and what Mozilla called "many enhancements and fixes for Windows Vista”.

Thunderbird's major updates always follow Mozilla's lead product, the open-source Firefox browser - a trend that continued with Version 2.0. Also like Firefox, Thunderbird will terminate security updates for the prior version - 1.5 - six months after the release of the 2.0 update, MacGregor said. Firefox 1.5, for example, is to fall off Mozilla's support on 24 April, six months to the day after its 24 October 24, 2006, debut. If the final version of Thunderbird 2.0 posts on 30 April, for example, Mozilla will drop 1.5 support on 30 October.

Thunderbird 2.0 RC1 can be downloaded for Windows, Mac OS X and Linux in 35 language-localised editions from the Mozilla site.