PST is Not Outlook File (ERROR)
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PST is not an Outlook file" error typically occurs when you try to open or import a PST file into Microsoft Outlook, but the file is not a valid PST file or it is corrupted.
PST files are Personal Storage Table files used by Microsoft Outlook to store your email messages, contacts, calendars, tasks, and other data. If you receive the "PST is not an Outlook file" error, it may indicate that the PST file is damaged, inaccessible, or not in the correct format.
Now to fix the damaged file of Outlook you can try PST Repair Tool. This utility repairs any level of corruption of the PST file and also, options to export repaired PST files such as PST, PDF, MSG, EML, HTML, and Office 365.