Repair Outlook Data File (.OST/.PST) to Fix Sync Errors

When Outlook starts freezing, stops syncing, or fails to open folders, a corrupted data file is often the culprit. Your emails, calendar, and contacts are stored in .OST (Offline Storage Table) or .PST (Personal Storage Table) files. If these files become damaged, Outlook can’t connect properly to Office 365 or your email provider. This guide shows how to repair Outlook OST/PST files safely without losing your mailbox data.

Understanding OST vs PST Files

• OST (Offline Storage Table): Used for Microsoft Exchange, IMAP, or Office 365 accounts. It syncs local copies of your mailbox.
• PST (Personal Storage Table): Used for POP3 or archived mailboxes stored locally on your computer.

When Outlook sync errors occur — especially “Cannot start Microsoft Outlook” or “Data file not found” — repairing these files is the first step.

Common Causes of Outlook Data File Corruption

• Unexpected system shutdown or power failure
• Large mailbox exceeding 10 GB
• Antivirus or backup software locking the file
• Faulty Outlook add-ins
• Improper termination of Outlook process

Method 1 – Repair OST/PST Using Microsoft’s Inbox Repair Tool (SCANPST.EXE)

1. Close Outlook completely.
2. Locate SCANPST.EXE on your computer:

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

3. Open the tool, click Browse, and select your .OST or .PST file:

   %localappdata%\Microsoft\Outlook

4. Click Start to begin scanning.
5. If errors are found, choose Repair.

This process creates a backup before repairing your data file. Once complete, reopen Outlook — it will rebuild and resync your mailbox automatically.

Method 2 – Delete and Rebuild the OST File

If you use Exchange or Office 365, you can safely delete the .OST file; Outlook will recreate it from the server.

1. Close Outlook.
2. Navigate to:

   %localappdata%\Microsoft\Outlook

3. Locate your OST file (example: outlook.ost) and rename it to outlook.old.
4. Restart Outlook — a new .OST file will be created automatically.

Method 3 – Create a New Outlook Profile

1. Go to Control Panel → Mail → Show Profiles → Add.
2. Enter your account details and click Next.
3. Select “Always use this profile” and delete the old one if it’s corrupted.

Method 4 – Use Microsoft Support and Recovery Assistant (SaRA)

Download the official SaRA Tool from Microsoft. It automatically detects corrupted data files, fixes profile errors, and reconfigures Outlook to restore connectivity with Office 365.

Advanced Tip – Reduce PST File Size for Better Performance

To avoid future corruption, compact large PST files:

1. File → Account Settings → Data Files → Settings → Advanced → Outlook Data File Settings.
2. Click Compact Now.

Related Articles

• Fix Outlook and Office 365 Sync Issues
• Fix Outlook Stuck on Synchronizing Subscribed Folders
• Email Setup and Configuration Guide

Conclusion

Repairing Outlook’s .OST and .PST files often restores full mailbox synchronization and prevents email loss. Always close Outlook properly, maintain smaller data files, and use the SCANPST tool or SaRA assistant for preventive maintenance. If errors persist, deleting the OST file or creating a new Outlook profile usually resolves all sync-related issues.