The set of folders cannot be opened. The file [filename].ost is not an Outlook data file
Microsoft Microsoft Office
Severity: CriticalWhat Does This Error Mean?
This error means Outlook found its data file (.ost) but the file is damaged or not recognized as a valid Outlook file. The .ost file is where Outlook stores your locally cached emails, calendar, and contacts. When this file gets corrupted, Outlook cannot load and shows this error instead of opening.
Affected Models
- Microsoft Outlook 2016
- Microsoft Outlook 2019
- Microsoft Outlook 2021
- Microsoft 365 Outlook
Common Causes
- The .ost data file is corrupted, often due to an improper shutdown or power loss while Outlook was open
- The .ost file path was changed or the file was moved to a different location
- A failed Office update left the installation in an inconsistent state
- Antivirus software scanned and quarantined or modified the .ost file
- The Outlook profile settings are pointing to a different or missing data file
How to Fix It
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Close Outlook completely. Find the .ost file (usually in C:\Users\YourName\AppData\Local\Microsoft\Outlook\) and rename it to something like email-backup.ost.old.
Renaming the file — not deleting it — tells Outlook to create a fresh .ost file. Your emails will re-download from the server. The old file stays as a backup.
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Reopen Outlook. It will detect that the .ost file is missing and create a new one automatically. It will then re-sync your emails from the server.
For large mailboxes, re-syncing can take 15–60 minutes depending on your internet speed. Let it run completely.
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If you prefer to repair rather than replace, run the ScanPST tool. Search for scanpst.exe on your PC, open it, browse to your .ost file, and click Start.
ScanPST can repair many types of .ost corruption. It is worth trying before renaming the file.
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If the error message mentions a specific file path, go to that path in File Explorer and check if the file exists. If it does not, delete the Outlook profile and create a new one.
A profile pointing to a missing file will always fail. Creating a fresh profile gives Outlook a clean starting point.
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Check your antivirus program's quarantine. If it grabbed the .ost file, restore it from quarantine and add the Outlook data folder to your antivirus exclusion list.
Some antivirus programs mistakenly flag large .ost files. Excluding the Outlook folder prevents this from happening again.
When to Call a Professional
If you cannot access critical business emails and the steps below do not work, contact your IT department. For Microsoft 365 accounts, all emails are stored on Microsoft's servers — your IT admin can reset your account and you will not lose data. For on-premises Exchange, an Exchange administrator may need to repair or resync your mailbox.
Frequently Asked Questions
Will renaming the .ost file delete all my emails?
No — not if your email is stored on a server. For Microsoft 365, Exchange, Gmail (IMAP), and most modern email accounts, all emails live on the server. Outlook will simply download a fresh copy when it restarts. Only POP3 accounts store emails exclusively on your PC — if you use POP3, proceed more carefully and back up your data first.
Where do I find the .ost file on my computer?
Open File Explorer and navigate to: C:\Users\[YourName]\AppData\Local\Microsoft\Outlook\ The AppData folder is hidden by default. To show it, click View in File Explorer and check the Hidden items box. You will see one or more .ost files named after your email address.
How long will it take Outlook to re-sync after I rename the .ost file?
It depends on how large your mailbox is and how fast your internet connection is. A mailbox with a few thousand emails typically takes 5–20 minutes. A large mailbox with years of emails and attachments could take an hour or more. Outlook will show a sync status bar at the bottom of the window while it downloads.