Mozilla has released Thunderbird 1.0 RC for testing.
Thunderbird is a next generation email client from The Mozilla Foundation. It offers a swathe of features, including spam filters, spell checkers, extension support, themes support, digital signing and built-in support for enterprise and government-level security options.
It supports IMAP/POP, HTML mail, saved search folders, message grouping, labels, quick search, RSS, smart addressbooks, receipts, message filtering, import tools and support for multiple accounts.
The client is available for multiple platforms, including Mac OS X. The Foundation invites feedback and testing of this release to help it develop the first complete release version of the application.
It's available for download here.