An OST file stores your mailbox data locally when using Cached Exchange Mode in Outlook. When this file becomes corrupted, you may experience crashes, missing emails, or sync errors. Corruption can happen due to sudden Outlook closure, disk errors, or profile issues. This article provides the steps to repair or recreate a damaged OST file to restore normal Outlook operation.
Key Takeaways: Repairing a Corrupted OST File
- Scanpst.exe (Inbox Repair Tool): Scans the OST for errors and attempts to repair the file structure if conversion is possible.
- Delete and Recreate OST: The most reliable fix involves removing the corrupted file and letting Outlook build a new one from the server.
- Control Panel > Mail > Show Profiles: The central location to manage your Outlook data files and create a new mail profile if needed.
Understanding OST File Corruption and Repair Options
The Offline Storage Table (OST) file is a local copy of your Exchange, Microsoft 365, or Outlook.com mailbox. It enables you to work offline and syncs changes when connected. Unlike a PST file, an OST is tightly linked to your specific mail profile and Exchange account. Direct manual repair of an OST is limited because it is a synchronized cache, not a standalone archive.
The primary symptom of corruption is Outlook failing to start or freezing. You might see error messages like “Cannot start Microsoft Outlook” or “The file C:\Users\…\Outlook.ost is not an Outlook data file.” The root cause is often file system damage from an improper shutdown, storage drive issues, or conflicts with antivirus software. The main repair strategy is to either use Microsoft’s Inbox Repair Tool on a converted copy or delete the file entirely and allow a fresh sync from the server.
Prerequisites for OST Repair
Before starting, ensure you have a stable internet connection and sufficient storage space. Your Exchange or Microsoft 365 account must be accessible. You will also need your account password. Close Outlook completely before performing any repair steps. Check your system’s Task Manager to confirm Outlook and related processes like SearchProtocolHost are not running.
Steps to Fix a Corrupted OST File
Follow these methods in order. Start with the Inbox Repair Tool, but be prepared to delete and recreate the OST file if errors persist.
Method 1: Use the Inbox Repair Tool (Scanpst.exe)
The Scanpst tool is designed for PST files. To use it on an OST, you must first create a copy and change its file extension. This method can fix minor structural issues.
- Locate and copy your OST file
Close Outlook. Open File Explorer and navigate to the OST location. The default path is C:\Users\[YourUsername]\AppData\Local\Microsoft\Outlook. Find the file named like “Outlook.ost”. Right-click it, select Copy, and paste the copy to your Desktop. - Rename the copied file
On your Desktop, right-click the copied file and select Rename. Change the file extension from .ost to .pst. For example, rename “Outlook – Copy.ost” to “Backup.pst”. Windows will warn you; click Yes to confirm. - Run the Inbox Repair Tool
Open the Run dialog with Windows key + R. Typescanpst.exeand press Enter. If it does not open, manually find it at C:\Program Files\Microsoft Office\root\OfficeXX or in the Office installation folder. Click Browse, navigate to your Desktop, and select the renamed .pst file. Click Start to begin the scan. - Complete the repair process
If errors are found, click Repair. After completion, the tool shows a summary. You now have a repaired .pst file on your Desktop. This file can be opened in Outlook via File > Open & Export > Open Outlook Data File for data recovery, but it is not a functioning OST.
Method 2: Delete the OST and Let Outlook Recreate It
This is the most effective solution. You will delete the corrupted file. Upon restarting Outlook, it will download a fresh copy from the mail server. All server-based items like emails, calendar events, and contacts will sync back.
- Close Outlook and open the OST folder
Ensure Outlook is fully closed. Open File Explorer to the OST folder at C:\Users\[YourUsername]\AppData\Local\Microsoft\Outlook. - Delete the corrupted OST file
Select the file named “Outlook.ost” or similar. Press the Delete key on your keyboard. You may move it to the Recycle Bin as a temporary backup, but a fresh file will be created. - Restart Outlook and wait for sync
Launch Outlook. It will detect the missing OST and begin creating a new one. A status message in the bottom-right will show “Processing” or “Updating Inbox.” This initial sync may take time depending on your mailbox size and connection speed.
Method 3: Create a New Outlook Profile
If corruption is linked to the profile itself, creating a new one is necessary. This builds a fresh OST file automatically.
- Open the Mail setup control panel
Press Windows key + R, typecontrol.exe /name Microsoft.Mail, and press Enter. Alternatively, search for “Mail” in the Windows Start menu. - Manage your profiles
In the Mail Setup window, click Show Profiles. Select your current profile and click Remove to delete it. Confirm the action. Do not worry; this only removes the local profile, not your server mailbox. - Add a new profile
Click Add. Enter a name for the new profile, like “Outlook 2024.” Click OK. The Add Account wizard will start. Enter your email address and password to set up your account again. Outlook will create a brand new, uncorrupted OST file for this profile.
Common Mistakes and Limitations to Avoid
Running Scanpst.exe on the Original OST File
The Inbox Repair Tool will not open a file with the .ost extension. Attempting to force it by selecting “All Files” in the browse dialog will result in an error. You must first create a copy and rename it to .pst as described in Method 1. Remember, the repaired output is a PST, not a replacement OST.
Deleting the OST Without a Stable Connection
If you delete your OST file while offline or with a poor connection, Outlook cannot recreate it from the server. You will be unable to access any cached mail data. Always ensure you have a working internet connection and can log into your email account before deleting the OST.
Assuming OST Repair Recovers All Data
The delete-and-recreate method only repopulates items still on the mail server. Any items that existed only in the local OST cache and were not synced to the server will be permanently lost. This includes draft emails saved locally or calendar entries created while offline if sync failed. Use the repaired PST from Method 1 to recover such items before deleting the original OST.
OST File Repair Methods Comparison
| Item | Inbox Repair Tool (Scanpst.exe) | Delete and Recreate OST | Create New Mail Profile |
|---|---|---|---|
| Primary Use Case | Minor file structure corruption, data recovery | Severe corruption, sync failures, crashes | Persistent profile-related errors |
| Data Source for Fix | Local copy of the OST file | Live Exchange/Microsoft 365 server | Live Exchange/Microsoft 365 server |
| Final Output | Repaired .pst data file for import | Fresh .ost file in same profile | New profile with a fresh .ost file |
| Time Required | Medium (scan/repair time) | Long (full mailbox resync) | Long (profile setup + full resync) |
| Risk of Data Loss | Low (original file is copied first) | High for unsynced local-only items | High for unsynced local-only items |
| Technical Skill Level | Intermediate | Beginner | Beginner to Intermediate |
You can now resolve OST file corruption by either attempting a repair with Scanpst.exe or forcing a fresh download from the server. For ongoing stability, ensure Outlook closes properly and consider running the Windows Disk Check tool. If corruption happens frequently, disable any antivirus real-time scanning of your Outlook data folder. A final advanced tip is to use the Outlook command outlook.exe /resetnavpane if folder views are still broken after the OST is recreated.