There is no limit. Apparently a crash will occur if the total length of the folder structure is around 240 characters. But in your case I would think it may well be damaged dbx files (outlook express uses one for each folder.) your might try running the import in Windows safe mode (that will disable all sorts of security and background tasks that might interfere or slow the process so it times out. *To restart the operating system in '''[http://en.wikipedia.org/wiki/Safe_mode safe mode with Networking]'''. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: [http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-including-safe-mode Windows 8], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-7 Windows 7], [http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-vista Windows Vista], [http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true" Windows XP], [http://support.apple.com/kb/ht1564 OSX]

Thanks for the reply. I'll try the safe mode. Is there a way to try to import just the rest of the dbx folders? Or Do I have to try again to import the whole thing?

Thunderbird can only do "the lot". there are programs that convert DBX files to their constituent EML files that can do things DBX by DBX, unDBX is one such https://code.google.com/p/undbx/

From my experience, using undx, you will loose the file structure of your emails (.eml). If you care about it, I would recommend importing the whole thing then filing what is actually useful to you.