A Microsoft Exchange hosted mailbox uses an .ost file to replicate and store local data in Outlook. Outlook will not open if the .ost Offline Folder file is damaged. In this case you can either try to repair the .ost file or delete it and resynchronise the data.

To repair the .ost file you must use the OST Integrity Tool: scanost.exe which located in your Program Files folder. You can find more information on the Microsoft support pages.

To create a new .ost Offline Folder file you first have to locate and delete the damaged .ost file. You can find more information about the location of the .ost file on the Microsoft support pages. After the .ost file has been deleted, you may create a new file to make Outlook work properly:

  1. On the Tools menu, click E-mail Accounts, click View or change existing e-mail accounts, and then click Next.
  2. In the list, click Microsoft Exchange Server, and then click Change.
  3. Click More Settings.
  4. Click the Advanced tab, and then click Offline Folder File Settings.
  5. Click Disable Offline Use.
  6. In the File box, type the path to the file you want to use as the new .ost file. The default file name is outlook.ost. If this file name already exists, you are prompted for a new name.
Outlook does not open because of damaged .ost file
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