Today I transferred email from Outlook Express 6 on Windows 2000 Professional SP4 to OS X Tiger's Mail.app. This is how I did it.
1. Download and install Mozilla Thunderbird. Current version is 18.104.22.168.
2. Launch and choose to import from Outlook Express. (Clean up the Outlook folders and empty the trash first.)
3. Copy the mail folder to the mac from the Thunderbird profile folder. I used a FAT32 formatted USB drive.
4. Add .mbox to all the files inside the mail folder you just copied that don't already have an extension. I used A Better Finder Rename for this.
5. Launch Mail.app and choose import from the File menu.
6. Select Netscape/Mozilla and go!
I hope this helps someone, because I had trouble following all the other tutorials. And I hope someone who goes thru this process fills in all the little details that I left out (such as the default directory of the mozilla profiles folder and the extension of the mozilla mail index, etc.)
Also check out these 2 links: