Thunderbird 1.5, which can be downloaded for free, has been retooled to offer improvements in four main areas: updates, security, RSS and podcasting.
"Thunderbird enhances the overall e-mail experience, adding antiphishing capabilities to help keep people safer, while also integrating and simplifying access to new technologies, such as RSS," Christopher Beard, Mozilla's vice president of products, said in a statement.
The e-mail client also features automated updates, designed to download security and product upgrades to people's systems and then to prompt them when ready for installation.
In another effort to bolster security, Thunderbird 1.5 is designed to push e-mail through a finer spam filter. Last fall, Mozillathat plugged several security holes.
Thunderbird 1.5 also aims to bolster its RSS support by letting people receive feed updates as e-mail messages. People can now access podcasts through a dialog box, which is tied to an application such as a Web browser or audio player.
Mozilla's new release also includes productivity enhancements, such as spellcheck as e-mail is being written and an ability to delete attachments from e-mail.
Thunderbird has been downloaded 18 million times since it debuted in December 2004, Mozilla said.