Your Outlook may freeze, fail to sync, or show errors like “Cannot start Microsoft Outlook” due to a damaged OST file. This file is a local copy of your Exchange or Microsoft 365 mailbox data. This guide provides the steps to rebuild the OST file, which forces Outlook to create a fresh, error-free copy of your mailbox data.
Key Takeaways: Rebuilding Your OST File
- Close Outlook and rename the .ost file: This prevents Outlook from using the corrupted file and triggers creation of a new one.
- Control Panel > Mail > Show Profiles > Remove: Deletes the mail profile, which is required before you can create a new OST.
- File > Account Settings > Add Account: Re-adds your email account, which automatically generates a new, healthy OST file.
Understanding the OST File and When to Rebuild It
An Offline Outlook Data File, or OST, stores a synchronized copy of your Exchange or Microsoft 365 mailbox on your computer. It enables you to work offline and can improve performance. This file is managed by Outlook and syncs with the server when a connection is available.
Problems occur when this local file becomes corrupted. Common signs include constant syncing errors, missing emails, calendar items that won’t update, or Outlook crashing on startup. A rebuild is a definitive fix that replaces the damaged local file with a new one downloaded from the server. Your online mailbox data remains safe during this process.
Prerequisites Before You Start
Ensure you have a stable internet connection and know your email account password. The rebuild process will require you to re-enter it. Also, verify you have sufficient disk space, as the new OST file will be the full size of your mailbox. Backing up any locally stored Outlook data files, like PST files, is also a good practice.
Steps to Rebuild a Corrupted OST File
The most reliable method is to remove your mail profile and add the account again. This ensures a completely clean start.
- Close Outlook completely
Right-click the Outlook icon in the system tray and select Close Outlook. Verify it is not running in Task Manager. - Rename the existing OST file
Open File Explorer and paste this path into the address bar: %localappdata%\Microsoft\Outlook. Find the file with your email address and the .ost extension. Right-click it, select Rename, and change the extension to .old. - Open the Mail control panel
Press the Windows key, type “Control Panel,” and open it. Set View by to Large icons and select Mail (Microsoft Outlook). - Remove your mail profile
In the Mail Setup window, click Show Profiles. Select your profile name and click Remove. Confirm the deletion. Click Apply and then OK to close the windows. - Start Outlook and re-add your account
Open Outlook. You will see the account setup wizard. Enter your full email address and click Connect. Follow the prompts to enter your password and complete the setup. Outlook will now create a new OST file and begin syncing your mailbox.
Common Mistakes and Limitations
Outlook Asks for a Password Repeatedly After Rebuild
This often points to an incorrect password or an issue with Modern Authentication. First, confirm your password is correct by signing into Outlook on the web. If the password is correct, close Outlook and open the Windows Credential Manager. Remove any stored credentials for Outlook or Microsoft Office. Restart Outlook and enter your password again when prompted.
The New OST File Is Not Downloading All Mail
By default, Outlook only syncs a limited period of mail to the OST file. To change this, go to File > Account Settings > Account Settings. Double-click your Exchange or Microsoft 365 account. Move the Mail to keep offline slider to All or your desired timeframe. Click Next and Finish, then restart Outlook to allow full synchronization.
Rebuilding Does Not Fix Missing Shared Mailboxes or Calendars
Shared mailboxes and calendars added as delegates are permissions-based and may not auto-populate. After rebuilding your primary OST, you may need to re-add these manually. For a shared mailbox, go to File > Account Settings > Account Settings. Select your account and click Change. Click More Settings > Advanced > Add, and enter the shared mailbox name.
OST File Management: Manual Deletion vs. Profile Rebuild
| Item | Manual OST Deletion/Rename | Full Profile Rebuild |
|---|---|---|
| Primary Use Case | Quick fix for sync errors when profile is otherwise healthy | Comprehensive fix for deep corruption, profile errors, or repeated failures |
| Process Complexity | Simple: close Outlook, rename file, restart | More involved: requires removing and recreating the mail profile |
| Impact on Settings | Preserves all Outlook settings, signatures, and rules | Resets some client-side settings; rules stored on server are preserved |
| Sync Time | OST resyncs from scratch, which can be lengthy for large mailboxes | OST resyncs from scratch, identical to manual deletion |
| Guarantee of Fix | Moderate; may not resolve underlying profile issues | High; provides a completely fresh start |
You can now resolve persistent Outlook errors by rebuilding the damaged OST file. The profile removal method is the most thorough solution for corruption. For future issues, remember you can run the Outlook inbox repair tool, SCANPST.EXE, on PST files, but it does not work on OSTs. A pro tip is to hold the Ctrl key while right-clicking the Outlook system tray icon to access the Connection Status window, which shows real-time sync details for your new OST file.