Using DataNumen Exchange Recovery to Fix OST File Errors

When you are using Exchange accounts, IMAP accounts, and Microsoft 365 accounts in Outlook, all your data are synchronized and saved in an offline folder (.ost) file. From time to time, you may encounter various errors in the OST file. Here we will list some symptoms.


1. When starting Microsoft Outlook, you get the following error message:

Cannot open your default email folders. The file xxxx.ost is not an offline folder file.

2. 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.

Note: In the above error messages, ‘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.

3. You encounter numerous conflict items in your offline folder (.ost) file.

4. You cannot open certain items in offline folder (.ost) file, when Outlook is working offline.

5. You can open folders in the offline folder (.ost) file, but cannot synchronize them with the Exchange server, or encounter various synchronization error messages shown in the synchronization log in the Deleted Items folder.

Precise Explanation:

There are 3 reasons that will cause these errors, as follows:

  • The OST file is damaged or corrupted, and cannot be recognized by Microsoft Outlook, so Outlook will report the error.
  • One or more messages in the OST file are damaged and the synchronization process cannot correct them.
  • The OST file is associated with a mailbox on the Exchange server. If for any reason, Microsoft Outlook cannot access the associated Exchange mailbox or start synchronizing the mailbox with the offline folders in the OST file, it will report the error. Some common examples are:

1. In Outlook, you haven’t set the email account for accessing the Exchange mailbox correctly.

2. In Outlook, you delete the email account for the Exchange mailbox.

3. In the Exchange server, the Exchange mailbox, or the email account for the Exchange mailbox is disabled or deleted.

4. There are communication problems between Outlook and the Exchange server.

5. You don’t have an Exchange email account at all. And your email account is based on POP3, IMAP, HTTP or mail servers other than the Exchange server. But you set your email account as Exchange-based by mistake.


If it is one or several erroneous messages that cause the error, then sometimes you can simply delete these messages to solve the error. Also, Microsoft provides the OST Integrity Check tool that can fix some minor synchronization errors as well. However, for most cases, the best solution to prevent data loss and further errors is using DataNumen Exchange Recovery, as below:

  1. Close Microsoft Outlook and any other application that may access the OST file.
  2. Find the OST file that has the problem. You can determine the file location based on the property displayed in Outlook. Or use the Search function in Windows to search for the OST file. Or search in the predefined locations for the 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 error-free Outlook PST file so that you can access all the messages and items with Outlook easily and efficiently.
  4. Back up the original OST file, for the sake of safety.
  5. Rename or remove the original OST file.
  6. Fix the error.
    1. If your Exchange mailbox is still valid, ensure the email account settings in Outlook are correct, and Outlook can connect to your Exchange server correctly. Then you can start Outlook and send/receive your emails on the Exchange mailbox, which will auto-generate a new OST file and synchronize its data with the Exchange mailbox. If this method doesn’t work, then please follow the instructions in (ii).
    2. If the instructions in (i) don’t work, then your current mail profile is incorrect, you should recreate a profile. Then re-synchronize your mailbox.
    3. If your Exchange mailbox doesn’t exist anymore, or you don’t have an Exchange mailbox at all, you can open the PST file generated in Step 3 directly and skip Step 7.
  7. Import the data recovered in step 3. After your OST file problem is solved, keep the new OST file for the 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 the required items to your new OST file as needed.