Error Solved: OST is not an Outlook Data File

OST files are created when you add an IMAP, Exchange, or Microsoft 365 account in Outlook. OST file stores the synchronized copy of your mailbox data on the mailbox server and allows offline access to the mailbox data. Sometimes, Outlook OST files get damaged, corrupted, or turn inaccessible due to various reasons, preventing you from accessing the emails and other items. When you try to access the damaged OST file, Outlook displays the following error message:

“Cannot start Microsoft Outlook. Cannot open the Outlook Window. The set of folders cannot be opened. The file c:\Users\USERNAME\Local\Microsoft\Outlook\abc@xyz.com.ost is not an Outlook Data File (.ost).”

Below we will mention the reasons that may trigger the OST is not an Outlook data file error and the solutions to effectively troubleshoot and resolve it.

Reasons behind the ‘OST is not an Outlook Data File’ Error

This error may occur due to several reasons. In most cases, it occurs when the OST file gets damaged due to integrity issues caused by one or more of the following reasons:

• Large OST file size (oversizing)
• Virus/malware intrusion
• Synchronization errors between Outlook and mailbox server
• System crash
• Hardware failure
• Outdated or corrupt/damaged Outlook installation
• Compatibility issues
• Sudden or forced system shutdown
• Force termination of Outlook
• Corrupt Outlook profile
• Faulty or incompatible add-ins

Methods to Resolve the OST is not an Outlook Data File Error

You can follow the methods discussed below to troubleshoot and fix the OST is not an Outlook Data File error.

Method 1: Update Outlook and Windows

If your Outlook or Windows is outdated, then it can lead to compatibility issues that may trigger such errors in Outlook. Thus, you can check and update both Outlook and Windows. The steps to update Outlook are as follows:

• Open Outlook, and go to File > Office Account > Update Options.
• From the available options, choose Update Now.

• If a new update is available, Outlook will start a wizard and download the latest updates. You need to complete the update wizard.

To update Windows, follow these steps:

1. Go to Settings > Windows Update.
2. Check for the Windows Updates.
3. If new updates are available, download and install them.
4. Then, restart the system.

After this, start Outlook and check if the OST is not an Outlook data file error is resolved. If not, move to the next method.

Method 2: Remove Faulty Add-Ins

Faulty add-ins can cause Outlook to crash, leading to errors, such as OST is not an Outlook data file. You can resolve this by removing the incompatible or faulty add-in from Outlook. The steps are as follows:

• Press Windows + R, type outlook.exe /safe, and press Enter.
• In Outlook 2007 and 2010, go to Outlook Options > Add-ins > Go. In Outlook 2013 and later, go to File > Options > Manage COM Add-Ins. This opens up a dialog box listing all installed add-ins.

• Uncheck all Add-Ins and click OK to disable them.

Restart Outlook normally. If you’re able to access Outlook emails and send/receive new messages, it means any faulty add-ins were causing the issue.

Now, enable one add-in at a time and then restart Outlook to check which add-in is faulty. Once found, uninstall or remove it from the Outlook.

Method 3: Disable Cached Exchange Mode

Disabling the Cached Exchange Mode also disables Offline access mode and directly connects Outlook with the mailbox server instead of saving changes in the OST file. This can help you overcome the error. However, you can enable the Cached Exchange Mode later. To disable Cached Exchange Mode, follow these steps:

• Navigate to File > Account Settings > Account Settings and then select your Outlook profile.
• Click Change and then uncheck the ‘Use Cached Exchange Mode‘ checkbox and click ‘Next.‘

• Click ‘OK‘ when a pop-up appears and then click ‘Finish.‘
• Restart Outlook and check if the error is gone.

Method 4: Repair Outlook Profile

OST files associated with a corrupt Outlook profile may become inaccessible and result in this OST file error. To resolve this, you can repair your Outlook profile. The steps are as follows:

• In Outlook 2010, 2013, 2016, or 2019, navigate to File > Account Settings > Account Settings.

• Click the Email tab and choose your account (Profile).
• Click on the Repair option.

• Follow the prompts in the ‘Repair Account’ wizard.

Once done, restart Outlook and check if the error is resolved. If not, you can create a new Outlook profile.

Method 5: Create a New Outlook Profile

When repairing the corrupt profile fails or does not work, you can create a new Outlook profile. The steps are as follows:

• Open Control Panel and click on ‘Mail.’
• Click Show Profiles.

• Click ‘Add’ and then enter the new profile name. Click ‘OK‘.
• In the ‘Add Account’ wizard, enter the email ID and password. Click ‘Next‘.
• Choose the new profile from the options and enable ‘Always use this profile‘ or choose the ‘Prompt for a profile to be used’ option and then click ‘OK.‘

• Restart Outlook.

Method 6: Recreate Outlook OST File

If the OST file is damaged or corrupted, you can recreate a new OST file by deleting or moving the inaccessible OST file to a different location. Follow these steps:

• Navigate to File > Account Settings > Account Settings.
• Click the Data Files tab and select the OST file associated with your profile.
• Click ‘Open File Location.’ This opens a new File Explorer window displaying the OST file location.

• Close Outlook and move the OST file to another location for backup.

Now open Outlook. This will automatically create a new OST file. After this, check all mail items. In case of missing mail items, you can use Stellar Converter for OST software to extract the items from your backup OST file and save them to PST format. Then, you can import this PST into your new OST file via the Outlook Import/Export wizard. You can also use the software to directly import the mail items from the OST file into an existing Outlook profile, Office 365, or live Exchange Server.

To Wrap Up

You can encounter the error OST is not an Outlook data file, if the OST file gets damaged or corrupted due to various reasons. However, you can resolve this by following the methods discussed in this post. However, if the error can’t be resolved, you can use a reliable OST to PST converter software to extract all the mailbox items from the inaccessible or orphaned OST file and save them to Outlook importable PST format. Once you have converted the mail items from your inaccessible OST to PST format, you can remove your current Outlook profile, set up a new Outlook profile, and configure your email account. Then, import the converted PST file into your newly set up Outlook profile via the Import Export wizard.

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About The Author

Ravi Singh

Ravi Singh is a Senior Writer at Stellar®. He is an expert Tech Explainer, IoT enthusiast, and a passionate nerd with over 7 years of experience in technical writing. He writes about Microsoft Exchange, Microsoft 365, Email Migration, Linux, Windows, Mac, DIY Tech, and Smart Home. Ravi spends most of his weekends working with IoT (DIY Smart Home) devices and playing Overwatch. He is also a solo traveler who loves hiking and exploring new trails.