We've written a program of our own to read email archives from Eudora and import them into a Cardbox database. You can read the story at http://cardbox.wordpress.com/2006/01/18/the-cardbox-email-archive/
If you'd like to use the program yourself, please email me on email@example.com. The program will not be an official part of Cardbox but I'm happy to make it available free on the basis that "if it works, it works, if it doesn't work, it doesn't".
If you want to discuss email and Cardbox in a wider context, please add replies to the discussion in Cardbox Talk.