In today’s era, Microsoft Outlook is one of the popular email clients used by many organizations and individuals. Sometimes Outlook users fail to access or read their important files, getting an error message that says “OST is not an Outlook Data File” which means that, Outlook cannot access or read the OST file. For instance, recently i have come across an user query where he was unable to open the OST file and got the following error message:

“Cannot start Microsoft Outlook. Cannot open the Outlook Window. The set of folders cannot be opened. The file c:\Users\USERNAME\Local\Microsoft\Outlook\ is not an Outlook Data File (.ost).”

The file was about the client's details shared by his boss. In a situation like this, the user panics and looks for an immediate solution to fix it. I would suggest users repair the mailbox manually first, if it doesn’t help, try to resolve it by automated tools. The main challenge is to resolve the error without any data loss.

fix OST file error

Why Outlook users face the “OST file is not an Outlook Data File” issue

The primary reason behind getting the "OST file is not an Outlook Data File" error is MS Outlook failure to synchronize the OST file with the MS Exchange server due to poor network connection. Some other circumstances that lead to this error are: 

  • Virus or malware attack
  • System crash or sudden power cut while running MS Outlook
  • Sudden closing of MS Outlook application

Manual approach to resolve this OST Error

We will try to reset the configuration of the Exchange server with MS Outlook to solve the error

  1. Go to Control Panel>>> All Control Panel items and select the Mail option.
  2. Select the Show Profile option and choose your Outlook profile.
  3. Next, click on Properties and then pick your Email Account.
  4. Then select Exchange and click on the Change button.
  5. Do not forget to enable Use Cached Exchange Mode checkbox.
  6. Click on More Settings under the Advanced tab and select Offline Folder File.
  7. Now, disable the offline use option and click on the Yes button.
  8. Again, go to the Offline Folder File settings and press the Ok button and Yes.
  9. Select the checkbox Use Cached Exchange Mode under Cached Exchange Mode Settings and press OK.
  10. Finish the process.

Note: Any mistakes while performing the manual arrangement would cause inconvenience in the future. Even after completing such a lengthy and complex process correctly, there is no surety that it will fix the error. Also, there is a risk of data loss. So, we suggest keeping a backup of your OST file before attempting any recovery method. 

Automated method

An effective solution to fix this error is to convert the OST file into PST file format by using a professional tool i.e. OST to PST conversion tool available online. It will help you create an accessible PST file from even corrupt and inaccessible OST files. It will help you solve the OST file is not an Outlook data file issue. It will give you a quick and reliable solution to your problem.

The OST file error message could be due to many possible reasons, and it could be this or anything else. Getting error messages not only hamper work productivity but also restrict data accessibility. To overcome such issues, we recommend opting software solutions to save your time rather than manually attempting different way outs to resolve such errors.