When you start Outlook, you may see the error message “The set of folders cannot be opened” followed by a reference to your mailbox data file. This error typically means the Offline Outlook Data File (.ost) is corrupted or has become out of sync with the Exchange or Microsoft 365 server. The .ost file is a local copy of your mailbox that Outlook uses to work offline or when the server is unavailable. This article explains the root cause of the error and provides step-by-step methods to repair or rebuild the .ost file so you can access your folders again.
Key Takeaways: Repairing a Corrupted OST File in Outlook
- File > Account Settings > Account Settings > Data Files: Locate the .ost file path to delete or rename it for a forced rebuild.
- Control Panel > Mail > Show Profiles > Data Files: Remove a corrupted profile and create a new one to generate a fresh .ost file.
- SCANPST.EXE (Inbox Repair Tool): Use this tool only for .pst files, not .ost files; do not attempt to repair an .ost directly with it.
Why Outlook Shows “The Set of Folders Cannot Be Opened”
The error occurs because Outlook cannot read the local .ost file correctly. An .ost file is created automatically when you configure an Exchange, Microsoft 365, or IMAP account in Outlook. It stores a synchronized copy of your mailbox data so you can work offline. When this file becomes corrupted due to a sudden shutdown, disk errors, or a large mailbox size, Outlook fails to open the folder hierarchy. The server-side mailbox is intact, but the local cache is broken. Deleting or renaming the .ost file forces Outlook to download a fresh copy from the server, which resolves the error without data loss.
Steps to Repair the OST File by Rebuilding It
The only reliable way to fix a corrupted .ost file is to delete it and let Outlook create a new one. Your actual mailbox data remains on the server. Follow these steps carefully to avoid losing any local-only data such as cached offline items.
- Close Outlook completely
Make sure Outlook is not running. Check the system tray near the clock for the Outlook icon. Right-click it and select Exit if present. If Outlook remains open, use Task Manager to end the Outlook process. - Open the Account Settings dialog
Press the Windows key, type Control Panel, and open it. Set View by to Large icons. Click Mail (Microsoft Outlook). In the Mail Setup dialog, click Show Profiles. Select your current profile and click Properties. Then click Data Files. - Find the OST file path and name
In the Data Files list, select the entry that says Outlook Data File or Offline Outlook Data File. Click Open File Location. This opens the folder where the .ost file is stored. Note the full file path and name, for example C:\Users\YourName\AppData\Local\Microsoft\Outlook\yourname@domain.com.ost. - Rename the OST file
Right-click the .ost file in File Explorer and choose Rename. Change the extension from .ost to .old (for example, yourname@domain.com.ost to yourname@domain.com.old). If you cannot see the file extension, enable it in File Explorer by clicking View > Show > File name extensions. - Restart Outlook to rebuild the file
Open Outlook. It will display a message saying that it is connecting to the server and creating a new data file. This process may take several minutes, depending on the size of your mailbox and the speed of your internet connection. All your folders, emails, calendar items, and contacts will reappear once the synchronization completes. - Delete the old OST file after verification
After confirming that all folders are accessible and data is intact, return to the folder where the .old file is stored and delete it. This frees up disk space.
If Outlook Still Has Issues After the Main Fix
In some cases, rebuilding the .ost file does not resolve the error, or the error returns after a short time. Check the following scenarios.
Outlook Profile Is Corrupted
If the error persists after rebuilding the .ost, the entire Outlook profile may be damaged. Create a new profile: open Control Panel > Mail > Show Profiles > Add. Give the profile a name, then configure your email account again. Set the new profile as the default by selecting Always use this profile and choosing the new profile name. Start Outlook with the fresh profile.
Error Returns After Every Rebuild
A recurring error indicates a deeper issue, such as a corrupted mailbox on the server or a damaged account configuration. Run the Outlook Inbox Repair Tool (SCANPST.EXE) on any associated .pst files if you use POP3 accounts. For Exchange or Microsoft 365 accounts, contact your IT administrator to run a server-side mailbox repair or check for mailbox size limits. Large mailboxes over 50 GB often cause .ost corruption on older Outlook versions.
Cached Exchange Mode Settings Cause Sync Failure
If you use Cached Exchange Mode, try disabling it temporarily. Go to File > Account Settings > Account Settings. Select your Exchange account and click Change. Uncheck Use Cached Exchange Mode. Restart Outlook and reconnect to the server. This forces Outlook to work in online mode and bypasses the local .ost file. If the error disappears, re-enable Cached Exchange Mode and rebuild the .ost again using the steps above.
Manual OST Rebuild vs Auto Account Setup: Key Differences
| Item | Manual OST Rebuild | Auto Account Setup |
|---|---|---|
| Trigger | User deletes or renames the .ost file manually | Outlook detects a missing or corrupted .ost and rebuilds automatically |
| Control | Full control over which .ost file is replaced | No user intervention; may rebuild the wrong file if multiple profiles exist |
| Data loss risk | Zero risk if steps are followed correctly; all data resides on server | Same risk level; auto rebuild also preserves server data |
| Speed | Moderate; requires manual navigation to the file location | Fast; happens in the background after profile creation |
| Best for | Users comfortable with File Explorer and Control Panel | Users who prefer a guided setup with minimal steps |
Conclusion
You can resolve the “The set of folders cannot be opened” error by renaming or deleting the corrupted .ost file and letting Outlook download a fresh copy from the server. This process restores full access to all folders without losing any mailbox data. After the rebuild, verify that your folders, emails, calendar, and contacts are complete. If the error returns, create a new Outlook profile or disable Cached Exchange Mode to isolate the issue. For persistent problems, ask your IT administrator to run a server-side mailbox repair using the Exchange Management Console.