Outlook is one of the most widely used email clients in the world, and its offline storage mechanism, the OST (Offline Storage Table) file plays a huge role in enabling users to work even without an active internet connection. However, OST files are prone to corruption due to various reasons such as abrupt shutdowns, syncing issues, system crashes, virus attacks, or oversized mailboxes.

A corrupt OST file can prevent Outlook from opening, cause missing emails, slow performance, or trigger repeated synchronization errors. Fortunately, there are several ways to repair or rebuild a damaged OST file without losing your data.

This guide covers everything you need to know about repairing corrupt OST files, including the root causes, manual repair options, and professional solutions.

What Is an OST File?

An OST file is a local copy of your mailbox stored on your computer. It is generated automatically when using:

• Microsoft Exchange/Office 365 accounts
• IMAP accounts
• Outlook.com accounts

The OST file allows you to access emails, calendars, contacts, and tasks even when offline. Once your system reconnects, Outlook syncs all changes back to the server.

Why Do OST Files Get Corrupted?

Before repairing a corrupt OST file, it’s important to understand the root cause. Some of the most common reasons include:

1. Abrupt Outlook or system shutdown

If Outlook or Windows restarts unexpectedly (due to a power failure or crash), ongoing data-writing processes may break, damaging the OST file.

2. Large or oversized OST file

As OST files grow beyond several gigabytes, they become more prone to corruption or slow synchronization.

3. Network or synchronization issues

Poor internet connectivity can cause incomplete syncs, leading to internal inconsistencies in the OST structure.

4. Bad sectors on the hard drive

If the portion of the disk storing the OST file becomes damaged, file corruption can occur.

5. Malware or virus infection

Malicious programs targeting email storage can damage OST file integrity.

6. Software conflicts or faulty add-ins

Conflicting Outlook add-ins, outdated antivirus tools, or faulty updates can disrupt OST operations.

Signs Your OST File Is Corrupt

You may be dealing with a corrupt OST file if Outlook shows any of these symptoms:

• Outlook won’t open or crashes repeatedly
• Emails or folders are missing
• “Cannot start Microsoft Outlook” error
• Sync errors like 0x800CCC0E, 0x8004010F, etc.
• Outlook takes too long to load
• Search results not showing correct items
• Strange behavior like duplicated items or folder mismatches

How to Repair a Corrupt OST File: All Possible Methods

Fortunately, OST corruption does not always mean data loss. Since emails, contacts, and calendar items are usually stored on the server for Exchange and IMAP accounts, you can often rebuild the OST file safely.

Here are the most reliable methods to repair or recover your OST data.

Method 1: Rebuild the OST File (Safest and Most Effective)

Recreating the OST file is the simplest solution because Outlook will download fresh data from the mail server. This method works best if you are using:

• Exchange
• Office 365
• Outlook.com
• IMAP accounts

Steps to Rebuild OST File

1. Close Outlook completely
2. Navigate to the OST file location:

C:\Users\\AppData\Local\Microsoft\Outlook\

3. Locate the OST file and rename it (e.g., add .old at the end).
4. Restart Outlook.

Outlook will automatically create a new, clean OST file and begin downloading mailbox data.

When to use this method:

• If OST file is heavily corrupted
• You have a stable internet connection
• Mailbox data is stored on the server

Method 2: Use Outlook’s Built-In Repair Tool — ScanOST (Legacy) / ScanPST

Older Outlook versions had a tool called ScanOST.exe, but Microsoft removed it from recent versions. Now, the ScanPST.exe tool (Inbox Repair Tool) is available and can repair PST files — and sometimes partially repair OST files.

Steps to Run ScanPST for OST Files

1. Close Outlook.
2. Locate ScanPST.exe:
   • For Office 365:

C:\Program Files\Microsoft Office\root\Office16\

For Outlook 2016:

C:\Program Files (x86)\Microsoft Office\Office16\

3. Double-click ScanPST.exe.
4. Browse and select your OST file.
5. Click Start to begin scanning.
6.Click Repair when prompted.

Limitations of ScanPST

• Cannot fully repair severely damaged OST files
• May restore only partial data
• Works slowly with large OST files
• Often fails when sync errors are complex

Use this method only when the corruption is minor.

Method 3: Enable Cached Exchange Mode Reset (Force Resync)

If the OST file is not massively corrupted, resetting Cached Exchange Mode forces Outlook to resync everything.

How to Reset Cached Exchange Mode

1. Go to Control Panel → Mail
2. Click Email Accounts
3. Select your email account → Change
4. Disable Use Cached Exchange Mode
5. Restart Outlook
6. Re-enable Cached Exchange Mode and restart again

This forces Outlook to rebuild the OST file internally.

Method 4: Create a New Outlook Profile

Sometimes, the corruption lies not in the OST file but in the Outlook profile itself.

Steps to Create New Outlook Profile

1. Open Control Panel → Mail
2. Click Show Profiles
3. Click Add
4. Enter a profile name
5. Add your email account
6. Choose Always use this profile
7. Restart Outlook

A new profile often resolves synchronization and corruption issues without manually handling OST files.

Method 5: Export Data to PST Before Rebuilding (If Outlook Still Opens)

If Outlook is still functional, even partially, you should export your mailbox data before attempting risky operations.

Steps to Export OST Data

1. Open Outlook
2. Go to File → Open & Export → Import/Export
3. Choose Export to a file
4. Select Outlook Data File (.pst)
5. Choose folders to export
6. Save the PST file

This ensures you don’t lose important emails during OST repair.

Method 6: Use Microsoft’s “Reset Folders” and “Reset Navigation Pane” Commands

Sometimes folder structures become corrupted. Outlook offers simple reset commands:

Reset Folder Structure

Run this command in Run dialog (Win + R):
outlook.exe /resetfolders

Reset Navigation Pane
outlook.exe /resetnavpane

Method 7: Use Professional OST Repair Software (For Severe Corruption)

If:

• Outlook won’t open
• OST file is 100% unreadable
• Sync is completely broken
• ScanPST fails
• The OST is orphaned (no access to the server)

Then your best option is professional OST repair software.

Reliable tools can:

• Repair corrupted OST
• Recover emails, calendars, contacts
• Extract data into PST format
• Restore orphaned OST files
• Recover deleted emails

Some examples include:

• Stellar Repair for Outlook
• Kernel OST to PST Converter
• SysTools OST Recovery

These tools are especially useful if the Exchange/IMAP server is not accessible anymore.

How to Prevent OST File Corruption

Though corruption can’t be prevented entirely, adopting good practices can reduce risk dramatically:

✔ Keep OST files below 10–20 GB

✔ Avoid abrupt system shutdowns

✔ Use high-quality antivirus tools

✔ Don’t overload Outlook with add-ins

✔ Keep Windows and Outlook updated

✔ Repair your disk regularly using CHKDSK

✔ Use SSDs instead of HDDs for better stability

Conclusion

A corrupt OST file can disrupt your ability to access emails, slow down Outlook, and interfere with your daily workflow. However, with the right repair method from simple rebuilds to advanced recovery tools, you can restore your mailbox safely and effectively.

In most cases, rebuilding the OST file is the easiest and most reliable fix. But for severe corruption or inaccessible accounts, professional OST repair tools offer the best chance of complete data recovery.

By understanding the causes, recognizing early signs of corruption, and applying the correct repair method, users can maintain a healthy Outlook environment and significantly reduce the risk of future issues.