Ask Max: Exporting from Apple Mail
I would like to move away from Apple Mail, either back to Eudora or Thunderbird. What's your opinion of each and where is the "export" function?
With 3,500 address and 350+ Mbytes of email re-entering is not an option.
I have been using Mail for years and, while recognizing the limitations, I like the overall integration that Mail has with OS X. This being said, my recommendation is going to be based on the opinions of some of my coworkers. Most non-Mail users around here are in favor of Thunderbird as opposed to Eudora. This is probably because Thunderbird is free and has a lot of new development around it. Thunderbird is missing an Apple Mail importer however. Eudora has the built-in option of importing Apple Mail but you will have to pay $49.95 or watch advertisements to use Eudora. Although it is not automated, Mozilla has some instructions on importing into Thunderbird found here: http://www.mozilla.org/support/thunderbird/faq#mail.app
Try following those instructions first and see if it works for you. If it fails to get you switched over then you'll have to decide if the price for Eudora is worth it.