From MozillaZine Knowledge Base
Qualcomm announced plans on October 11, 2006 to produce a open source version of Eudora based on Thunderbird, and that once it was available to stop selling Eudora commercially. On Sept 13, 2010 they released Eudora OSE, which had initially been called Eudora 8. Its available for Windows (XP and later versions) OS X (10.4.x and later) and Linux.
Eudora OSE is based on Thunderbird 3.0.4 source code (3.0.4 was released March 30, 2010) and includes a Penelope extension. Almost all of the Eudora specific features are in the Penelope extension. None of the commercial Eudora source code is used in either deliverable. Qualcomm had five developers and a project manager working full time on the project.
Bugs are reported in Mozilla's Bugzilla database just like Thunderbird's. If you install Eudora OSE it will automatically find and use your default Thunderbird profile (if it exists).
Much of the interest in Eudora OSE in the MozillaZine forums has died off due to the slow pace of the project and people realizing how little of what made classic Eudora unique was kept. There doesn't seem to be any information available on future plans. A Eudora clone called Odysseus was developed by another company as a commercial product at the same time. Its now called MailForge. It originally was a cross platform product that supported Windows and both PowerPC and Intel Macs. The latest version uses Cocoa and only supports Intel Macs.