OST to PST Converter

Error Solved: OST is not an Outlook Data File

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    Summary: When the OST file becomes inaccessible and Outlook fails to access or read the OST file, it displays an error message, such as OST is not an Outlook Data File. In this blog, we have discussed the reasons behind the error and the methods to resolve this error without any data loss.

    Outlook OST files may get damaged or turn inaccessible due to various reasons. When this happens, Outlook cannot access or read the OST file and 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).”

    Microsoft Outlook

    As a result, users lose access to their Outlook account and mailbox data. This may occur anytime and without warning in Outlook 2010, 2013, 2016, or 2019.

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

    The 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 size (oversizing)
    • Virus/malware intrusion
    • Synchronization errors between Outlook and Exchange servers due to poor network connection or damaged encryption key
    • System crash
    • Sudden or forced system shutdown
    • Force termination of Outlook
    • Corrupt Outlook profile

    Methods to Resolve the OST File Error

    Unlike PST Outlook data files, OST can’t be opened or accessed from other Outlook accounts. OST file requires authorization from the linked MAPI profile to allow access to the mailbox data. Thus, you need an OST to PST converter software, such as Stellar Converter for OST, to extract the mailbox items, such as emails, attachments, contacts, notes, tasks, calendar items, etc. from inaccessible OST files to PST format.


    Once you have converted the inaccessible OST to PST format, remove your current Outlook profile from Outlook. Setup a new Outlook profile and configure your email account. Then import the converted PST file into your newly setup Outlook profile via the Import Export wizard. You may also use the software to directly import the mail items from inaccessible OST files into an existing Outlook profile, Office 365, or live Exchange Server.

    This is the easiest and most convenient way to resolve the error and restore the mail items from inaccessible OST files with complete integrity and without any risk of data loss.

    However, you may follow the manual methods discussed below to troubleshoot and fix the OST is not an Outlook Data File error.  

    IMPORTANT NOTE: Before applying these methods, back up your current OST file by copying it to another disk volume or external media. If the manual methods fail to fix the error, you can always use Stellar Converter for OST software to restore mail items from the backup OST file.

    Method 1: Update Windows and Restart

    Some programs may stop working or display an error if Windows updates are pending for installation. Go to ‘Settings > Windows Update‘ and check for the Windows updates. Install the pending updates and restart the system. Then start Outlook and check if the error is resolved. If not, skip to the next method.

    Method 2: Repair Outlook Profile

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

    • In Outlook 2010, 2013, 2016 or 2019, navigate to File > Account Settings > Account Settings.
    Account Settings
    • Click the Email tab and choose your account (Profile).
    • Click on the Repair option.
    Repair Option
    • Follow the prompts in the ‘Repair Account‘ wizard.
    Repair account

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

    Method 3: Create a New Outlook Profile

    To create a new Outlook profile and fix the OST is not an Outlook data file error, follow these steps:

    • Open Control Panel, search and click on ‘Mail.
    • Click Show Profiles.
    Control Panel
    • Click ‘Add’ and then enter the new profile name. Click ‘OK‘. 
    • In the ‘Add Account’ wizard, enter a name, 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.
    Click OK
    • Restart Outlook and choose the new Outlook profile.

    This should resolve the Outlook OST file error.

    Method 4: Disable Cached Exchange Mode

    Disabling the Cached Exchange Mode also disables Offline Mode. This can help you fix the issue but you won’t be able to use Outlook when offline. However, you can enable 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.
    Disable Cached Exchange Mode
    • Click ‘OK‘ when a pop-up appears and then click ‘Finish.
    • Restart Outlook and check if the error is gone.

    Method 5: Recreate Outlook OST File

    You can recreate a new OST file by deleting or moving the inaccessible OST file to a different location. Follow these steps to find the Outlook OST file and then delete or move it from the default location.

    • 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.
    Recreate Outlook OST File
    • Close Outlook, right-click on the OST file, and choose ‘Delete.’

    Now open Outlook. This will automatically recreate a new OST file and fix the error.

    NOTE: This works only if your email profile is active on the mail server.

    Method 6: Use Microsoft Support and Recovery Assistant

    Microsoft Support and Recovery Assistant runs a few tests on your system, detects what’s wrong with the application or the system, and provides the best solutions to fix the identified issue. You can download Microsoft Support and Recovery Assistant setup and launch it. Then follow these steps:

    • Click ‘Install.’ This will download the required files and install the Microsoft Support and Recovery Assistant.
    • After installation, Microsoft Support and Recovery Assistant will open. Click ‘I agree.’
    Click I agree
    • Select Outlook from the options and click ‘Next.’
    Select Outlook
    • Select Outlook Won’t Start or Other problems and click ‘Next.’
    • Click ‘Yes‘ and then click ‘Next.‘ Follow the wizard and use options to troubleshoot and fix the problem with the Outlook OST file.

    To Wrap Up

    Offline Folder file (OST) may get damaged or corrupt due to various reasons. However, you can resolve this by following the manual methods discussed in this blog. However, the best way to resolve OST and Outlook-related errors, such as OST is not an Outlook data file’, is by using a reliable OST to PST converter software. The software can help you prevent data loss and extract all mailbox items from the inaccessible or orphaned OST file to Outlook importable PST format. In addition, you can also export and restore the mail items from inaccessible OST files directly to an existing Outlook profile, Office 365, or live Exchange.  

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    About The Author

    Eric Simson linkdin

    Eric Simson is an Email Platform Consultant and is associated with Stellar Data Recovery from last 6 years. He writes about the latest technology tips and provides custom solutions related to MS Outlook, MS Exchange Server, Office 365, and many other Email Clients & Servers.


    1. If this is a MS 365 account, check that the user licence hasn’t been unassigned. Have had this happen a few times with one particular tenant.

    2. I am expecting this kind of robust tool fix Outlook OST file error from Microsoft as a free assistance.

      1. Don’t worry!

        OST file error can be eliminated using manual tips. But, users generally avoid technical jargons.

        To simplify troubleshooting experience, we are providing a useful solution to fix error i.e. ‘OST is not an Outlook Data File‘ smoothly.

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