When using Microsoft Outlook to open or synchronize an offline folder (.ost) file, you see the following error message:

Unable to expand the folder. The set of folders could not be opened. Errors could have been detected in the file xxxx.ost. Quit all mail-enabled applications, and then use the Inbox Repair Tool.

where ‘xxxx.ost’ is name of the offline folder (.ost) file created by Outlook when it is working with the Exchange mailbox offline. You may not be familiar with the file as it is created implicitly.

Precise Explanation:

When your OST file becomes corrupt or damaged, and cannot be processed by Microsoft Outlook, it will report this error.


To solve this error and prevent data loss, please do as follows:

  1. Close Microsoft Outlook and any other applications that may access the OST file.
  2. Find the OST file that causes the error. Based on the information in the error message, you can easily find that file. You can also use the Search function in Windows to search for the OST file.
  3. Recover the offline data in the OST file. The Exchange OST file contains offline data, including the mail messages and all other items, in your Exchange mailbox, which are very important to you. To recover and rescue these data, you must use DataNumen Exchange Recovery to scan the OST file, recover the data in it, and save them into an Outlook PST file so that you can access all the messages and items with Outlook easily and efficiently.
  4. Backup the original OST file. For the sake of security, you’d better backup it.
  5. Rename or delete the original OST file that causes the problem.
  6. Fix the error. Make sure the email account settings in Outlook are correct, and Outlook can connect to your Exchange server correctly. Then restart Outlook and send/receive your emails on the corresponding Exchange mailbox, which will let Outlook create a new OST file automatically and synchronize its data with the Exchange mailbox.If this method doesn’t work, then your current mail profile is incorrect, you must delete it and create a new one, as follows:
    • 6.1 Close Microsoft Outlook.
    • 6.2 Click Start, and then click Control Panel.
    • 6.3 Click Switch to Classic View if you are using Windows XP or higher versions.
    • 6.4 Double-click Mail.
    • 6.5 In the Mail Setup dialog box, click Show Profiles.
    • 6.6 Select one of the incorrect profile in the list and click Remove to remove it.
    • 6.7 Repeat 6.6 until all incorrect profiles have been removed.
    • 6.8 Click Add to create a new profile and add email accounts according to their settings on the server.
    • 6.9 Start Outlook and re-synchronize your Exchange mailbox, you will find the problem disappear.
  7. Import the data recovered in step 3. After your OST file problem is solved, keep the new OST file for the Exchange mailbox open, and then open the PST file generated in step 3 with Outlook. As it contains all the recovered data in your original OST file, you can copy required items to your new OST file selectively.