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.

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:

There are two reasons that will cause this error, as follows:

  • The OST file is damaged or corrupted, and cannot be recognized by Microsoft Outlook, so Outlook will report this error.
  • The OST file is associated with a mailbox on the Exchange server. If for any reasons, Microsoft Outlook cannot access the associated Exchange mailbox or start synchronizing the mailbox with the offline folders in the OST file, it will report this 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 Exchange server, the Exchange mailbox, or the email account for the Exchange mailbox is disabled or deleted.

4. There are communication problems between the 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 Exchange server. But you set your email account as Exchange-based by mistake.


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

  1. Find the OST file that causes the error. Based on the information in the error message, you can easily find that file. You may also use the Search function in Windows to search for the OST file.
  2. Rescue 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 vital to you. To 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.
  3. Backup the OST file. For the sake of security, you’d better backup it.
  4. Rename or delete the original OST file that causes the problem.
  5. Fix the error.
    • 5.1.

    If your Exchange mailbox and email account are still valid, then you should make sure 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 make Outlook to create a new OST file automatically and synchronize its data with the Exchange mailbox.If this method doesn’t work, then please follow the instructions in 5.2.

  6. 5.2. If your Exchange mailbox or email account doesn’t exist any more, or you don’t have an Exchange email account at all, or the instructions in 5.1 don’t work, then your current mail profile is incorrect, you should delete it and create a new one, as follows:
    • 5.2.1 Click Start, and then click Control Panel.
    • 5.2.2 Click Switch to Classic View if you are using Windows XP or higher versions.
    • 5.2.3 Double-click Mail.
    • 5.2.4 In the Mail Setup dialog box, click Show Profiles.
    • 5.2.5 Select one of the incorrect profile in the list and click Remove to remove it.
    • 5.2.6 Repeat 5.2.5 until all incorrect profiles have been removed.
    • 5.2.7 Click Add to create a new profile and add email accounts according to their types.
    • 5.2.8 Start Outlook, you will find the problem disappear.