Ok, just spent 3 hours on the phone with Google tech support. Thunderbird on current MAC OSX seems to store mail in something other than the mbox extension, which is what Google Email Uploader wants to see. Outlook express with Google Apps Migration For Microsoft Exchange would be the best solution, but for some reason my Windows 7 machine was not permitted to access the legacy server - not in any browser or Outlook. In the end we utilized Apple Mail, then used the Uploader. Works, but apparently the folder structure isn't maintained (still uploading now), but at least it's where it's supposed to be. So not a great solution for multiple accounts.