There is a trick to this: I have switched since from an Outlook Exchange provider to an regular IMAP/POP3 email provider. As i did not need the old emails at that time, I've just saved the .ost file (just in case) ... you guessed ... now I need some of the old email.
Thanks for your help!
Edit 1: I found some good pointers here: http://www.geekstogo.com/forum/Converting-Outlook-OST-files-to-pst-t188080.html ... I'm still investigating ...