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You will probably have followed a zillion links before you got to this page and the only reason I am blogging about it is because most of these links will provide you with useless information or point you to software which will only export the first few files.
If you, like I, have collected thousands of emails in your trusted .pst folders on Microsoft Outlook and you (like I) are moving to another platform such as Apple, you will quickly notice that taking your mail with you is not such an easy thing.The easiest route could be through Outlook express. But installing that becomes more and more difficult and it does not handle .pst files that are stored on disk.
So we go for the thunderbird route. Install thunderbird on your PC running outlook. Click Tools->Import and select Outlook. Go for some coffee.
Now you should have all your stuff in thunderbird msf format, which will not do much good. Go to import/export tools and download the extension that will allow you to export in mbox format.
Right click the folder “Outlook Import” that you just imported and export everything in mbox format including a nifty directory structure.
Enjoy!
Related articles
- Importing Email into Mozilla Thunderbird (brighthub.com)
Lukassen's Blog 
There is no need to convert to mbox if you only want to import into Apple Mail (under Snow Leopard/Mail 4.5 and Lion/Mail 5.2 at least). After copying the Thunderbird profile, click File > Import Mailboxes… > Thunderbird > Continue > and you’ll be prompted for the profile location.
I have tried both methods (mbox conversion and Thunderbird profile) to import into Apple Mail. There is a large percentage of the folders and subfolders that import properly. However, there are specific folders that always import empty… I have tried many different twists to the process, but the result is always the same.
Any suggestions would be appreciated.