Konvert to maildir...
Evolution has a built-in function to convert a folder between 'mbox' and 'maildir' formats. Just right-click on the folder and choose 'maildir' instead. The mails can then be found in '~/evolution/local/"foldername"/mbox/cur|new'.
Hope this helps, it worked for me anyway...
Cheers, Christian
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People who don't make mistakes, usually do nothing at all...