A replica of user's Exchange mailbox is called Offline Storage Table (OST) file. This file contains user's emails, contacts, and other details similar to what is available on the Exchange Server. The benefit of OST file is that it helps you to work offline, which means even the Outlook's connection to the Exchange Server is broken, you can work offline. While it is the benefit of OST file, you may come across a situation when this file becomes inaccessible. The OST file can be inaccessible when user's mailbox is deleted from Exchange server, user's mailbox is corrupt, or the Exchange server database is damaged. In such a situation, Outlook cannot recognize the OST file when you set it to work in an Offline mode, and you get the below error message:
"Cannot open your default email folders. The file xyx.ost is not an offline folder file."
The xyz.ost is the name of OST file, which has become inaccessible.
You see the above listed error message due to below reasons:
When the aforementioned error message appears, you get into a situation of data loss where you have no more access to emails, etc.
Perform the below listed steps to fix the OST error message:
You can use an efficient software to convert OST to PST. The software will scan the corrupt or orphan OST file, fetch all data from it, and then you will be able to save that data in a PST file. You can save this PST file anywhere you want on your PC. After the PST file is created, start MS Outlook and import the newly created PST file with it. This way you can get back the inaccessible data. One of the well-known OST to PST Converter software, which is known to deliver good results in less time.