FAQ-Wiki

A:

Microsoft Outlook has no function with which to import data from *.ost files. To open an *.ost file in Outlook, the Exchange Server and Windows Domain Controller must be working and available for user authentication in both of the respective services. Only after that is it possible to transfer the contents of an .ost file that is open in Outlook to a .pst file that is also open in Outlook.

If the Exchange Server or Windows Domain Controller isn't accessible or doesn't work, or if the user can't be authenticated, access to the data will be denied and Outlook won't open the .ost file.