If you are getting a message that the PST is not an Outlook Data File, then it seems that your Outlook data file has damaged. Once Outlook data file gets corrupt then Outlook fails to recognize it and produces this error. You can repair your corrupted PST file using Scanpst.exe utility. It’s a free utility suggested by Microsoft Outlook. However, sometimes, scanpst is not helpful in repairing the PST file if it is severely corrupted. In that scenario, you can use an Outlook PST Repair tool.