How to Fix Microsoft Outlook OST File Cannot Be Opened Issue?

“Microsoft Outlook OST file cannot be accessed or opened” happens when there’s a glitch between local computer and Exchange/Microsoft 365 server. This usually happens due to damaged Outlook profile and OST file, Exchange Server connectivity problems, storage failures, or permission-related issues.

In simple terms, if your Outlook is connected with IMAP accounts, then it depends on OST file to maintain a local copy of your mailbox, and when it becomes inaccessible or orphaned, Outlook cannot read mailbox data, show synchronization errors, or cause complete mailbox inaccessibility.

This write-up will explain the answer to “How to fix “Microsoft Outlook OST file cannot be opened” error?” using 8 different manual and automated techniques.

Why Does Outlook OST Cannot Be Accessed Error Occur?

Microsoft Outlook OST file cannot be accessed for various technical reasons. It can be a sudden shutdown while the file is syncing or a disconnection from the Exchange server. Understanding the root causes is important to pinpoint the issue and apply the right solution.

• Sudden Shutdowns: Abrupt shutdowns or force-closing during syncing.
• Mailbox Overload: Exceeding the 50 GB size limit makes OST files unstable.
• Damaged OST File: Corrupt or orphaned OST files are a major reason why the OST file cannot be accessed.
• Corrupted Outlook Profile: The MAPI profile (connecting the Outlook client to the server) is misconfigured.
• Add-ins Interference: Incompatible third-party plugins are disturbing Outlook’s boot process.
• Connection loss with Exchange Server: Network interruptions during Exchange server synchronization can damage the process.

How to Fix Microsoft Outlook OST File Cannot be Opened?

This error can be resolved using right technique. We have explained simple to technical techniques one by one to fix the “Outlook OST file cannot be accessed” issue.

Method 1: End Task of Outlook

1. Close your Outlook completely.
2. Right-click on the taskbar menu and choose Task Manager.
3. Now locate Communicator, Lync, Outlook, or Ucmapi, if any process is running, right-click on it, choose End Process option.
4. Try to restart application.

Method 2: Enter Safe Mode and Disable Add-ins

Sometimes third-party add-ins create issues in Outlook, so we can launch the application in Safe Mode to fix the “Outlook OST cannot be accessed” error.

1. Use Windows Key + R, and type: outlook.exe /safe.

   

2. MS Outlook will open in safe mode.
3. After launching your application, if it successfully fetches data, then your error is resolved.
4. It means third-party add-ins are conflicting with Outlook, so disable them by choosing Options in Outlook >> head to the Add-ins >> Go icon >> and Uncheck all add-ins.

   

Method 3: Fix “OST File Cannot be Accessed” Issue by Resetting Navigation Pane

Sometimes, customizations made in the navigation pane make Outlook unable to open OST file. To remove these customizations, open the Run using Windows + R keys, and type Outlook.exe /resetnavpane and hit on the Enter button.

Method 4: Recreate a New Profile

Users can replace old damaged file by recreating a new profile by following mentioned steps:

1. Close application and open “Control Panel”.
2. Locate the “User Accounts” > “Mail” >> and select the “Show Profiles” button.
3. Then, click on the “Add” icon.
4. Type name of this new profile and choose “OK” button.
5. Provide username, email address and password.
6. After completing the sign-in, restart Outlook.

Method 5: Delete Corrupted OST File and Create a New One (The Ultimate Fix)

OST files are basically the mirror of your live mailbox on the Exchange server. So, if Outlook OST file cannot be accessed in Windows 11, you can delete the corrupted OST file and generate a new one from the server.

Note: You can use the method only if your Exchange/Microsoft 365 account is currently active/accessible. It is advised not to delete the OST file if your account is inaccessible.

Steps to Rebuild OST File:

First, close Microsoft Outlook completely.

1. Hit “Windows + R” key >> copy-paste %localappdata%\Microsoft\outlook >> Hit Enter.
2. Find the file with the .ost extension (named after your email address).
3. Click the “File” >> select “Rename” >> modify the extension from .ost to .old.
4. Finally, restart Outlook.

Following this will automatically connect to the Exchange server. Also, it will create a fresh OST file and download your mailbox data.

But what happens when your Microsoft 365 or Exchange account gets disabled/deleted, or your company shifts to a new domain? How will you be able to open file in Outlook 365?

In such scenarios, the “Microsoft Outlook OST file cannot be opened Reddit” community suggested: Fix the orphaned OST file and convert it into a PST file.

How to Fix an Orphaned OST File?

When a server account no longer exists, the OST file is considered “Orphaned”. You will find my IT blogs claiming that deleting and rebuilding the OST file can fix “Cannot start Microsoft OST file cannot be accessed.”

However, it is not possible! Because if the server account is deleted, your Outlook cannot build the file. Also, OST files are encrypted. It means you cannot easily copy an orphaned file to another computer or import it into a new Outlook profile.

And since Microsoft doesn’t provide a built -in tool to convert. You are required to hire a top-rated OST converter tool.

Download and install a trusted converter like SysTools OST to PST Converter.

Method 6: Turn off Cached Exchange Mode

1. Shut down Outlook and head to the “Control Panel”.
2. Click on the “Mail” option and then “Email Accounts”.
3. Press the “Email” tab.
4. Now click on the “Exchange Account” and choose “Change” option.
5. Next, uncheck the “Cached Exchange Mode” option and select the “Next” button.

Method 7: Repair Damaged Outlook Profile

Repairing Outlook profile can fix the OST file cannot be accessed issue:

1. Navigate the location “Control Panel > Programs and Features”.

   

2. Choose “Microsoft Office (or Outlook)” from the list and choose “Change” button.

   

3. Select “Quick Repair” and then “Repair” option.

Method 8: Disable Compatibility Mode

If your Outlook is running in compatibility mode, then it will appear Outlook OST file cannot be accessed or opened. It will useful for newly upgraded to newer version of Windows or Outlook, because compatibility mode often enabled automatically.

1. Locate a shortcut for MS Outlook.
2. Right-click on Shortcut and choose “Properties”.
3. Choose “Compatibility” tab.
4. Uncheck the “Run this program in compatibility mode for” box.
5. Hit the “Apply” >> “OK” button to save the changes.
6. Finally, double-click on the Outlook shortcut to start the program.

Author’s Final Words

This write-up outlined 8 different techniques to fix the “Outlook OST file cannot be accessed and opened” error. It happens when your Outlook profile and OST file damaged, server connectivity is disrupted, mailboxes are overloaded, add-ins interfere, or storage failures or permission issues. Users can fix the issue using manual technique; however, if OST file is severely damaged, then use the automated solution. For any queries, you can connect with us.

FAQs (Frequently Asked Questions)

Q.1 Will I lose my emails if I delete my OST file?

Absolutely Not! You will not lose any emails, calendars, and contacts till they’re synced to the Exchange/Microsoft 365 server.

Q.2 What is the difference between repairing an OST and recreating it?

The difference is that Microsoft’s Inbox Repair Tool (SCANPST.EXE) is specifically designed for PST files. Though it can OSTs, Microsoft clearly suggests recreating the file.

Q.3 Can I change where my OST is saved?

Yes! By creating a ForceOSTPath string value in the Registry under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook, you can change the destination for future OST files

Q.4 Microsoft Outlook OST file cannot be opened in Windows 11?

To fix this, you can try the following potential fixes:

• Unlocking the OST file in the Task Manager.
• Booting into Safe Mode and Disabling Add-ins.
• Converting orphaned OST files into PST format.

Q.5 What is the default Outlook OST file location?

The default OST file location for Outlook is the C:\ drive