“Cannot start Outlook OST is not an Outlook data file” error occurs when Outlook rejects OST file and prevents access to your mailbox data. It usually happens when users attempt to import, open, attach or restore OST file directly in Outlook.
MS Outlook application creates OST file to store complete mailbox data, when profile is connected to server accounts. It can hold emails, contacts, notes, journals, calendars, and other data smoothly just like PST format.
“Is not an Outlook data file .ost” error generally occurs when users try to open orphan or disconnected OST file in Outlook, because orphan file cannot be accessed in Outlook. Not only this, but multiple other reasons may trigger error.
This write-up will help users understand everything, including the meaning of this error, why it happens, its symptoms, and techniques to resolve it.
What Does “OST Is Not an Outlook Data File” Mean?
When MS Outlook is connected with Microsoft Exchange Server, Microsoft 365, Outlook.com, or IMAP accounts, it stores data in OST format. It is just local copy of your mailbox data.
The main reason behind “cannot start Outlook OST is not an Outlook data file” error is Outlook cannot recognize, validate, or use the selected OST file as a valid mailbox data file.
It doesn’t always mean that your OST file is invalid. It just means Outlook may be unable to access it due to profile mismatches, corruption, and damaged file structure.
Why Does Outlook Say OST Is Not an Outlook Data File?
Let’s understand the root cause of this problem so that we can choose right technique:
- Trying to Open Orphan OST: This is the major cause. If users are trying to directly import or open disconnected (orphan) OST files, then Outlook starts displays this error. Because MS Outlook doesn’t open or support disconnected files.
- Corrupted OST File: Yes, corruption is another major cause of this error. If your file is damaged due to any Sudden system shutdowns, power failures, hard drive issues, oversized files etc.
- Outlook Profile Mismatch: Every OST file is associated with an Outlook profile. If user reinstall Outlook, creates a new profile, migrates to another machine, or changes Exchange accounts, your existing OST file becomes orphaned and can no longer be configured with mailbox.
- Incomplete OST File: Copied OST file during synchronization may become incomplete.
- Damaged Outlook Profile: Not only data files, but corrupt profile can also affect performance.
Common Symptoms Associated with “OST is Not an Outlook Data File” Error
- Outlook refuses to attach OST file.
- Mail folder disappears.
- Synchronization process failures
- Outlook crash during startup.
- Missing emails and other data.
- Offline mailbox inaccessible
How to Fix “OST Is Not an Outlook Data File” Manually?
We can resolve it by checking the correct file extension, creating a new profile, rebuilding new OST file, repairing Outlook profile, checking basic connectivity, and using alternate solution.
Important Note: Manual Method only works when OST file is connected with Outlook profile. Means users cannot restore data from damaged or orphaned OST files using any advanced solution. You can use SysTools OST Recovery Tool to restore complete data from a damaged data file into a healthy format such as PST, PDF, EML, MSG, HTML and Office 365. It can handle all types of corruption from minor to severe. You should try free demo edition to restore complete mailbox including permanently deleted emails and fix “OST is not an Outlook data file” error.
Method 1: Basic Troubleshooting
- Make sure you are selecting only .ost extension.
- Also, don’t open OST file using Open Outlook Data File option.
- Restart your Outlook application or machine.
- Verify connected email account with OST file is still active.
- Make sure internet connection is working and stable.
- Also, verify Exchange server availability, Microsoft 365 service status, and Account authentication.
Method 2: Create a New Outlook Profile
If your Outlook profile is damaged, then recreate it by following these steps:
- Close your Outlook.
- Open “Control Panel”.
- Choose the “Mail” >> “Show Profiles” option.
- Click the “Add” button and provide a profile name.
- Configure your email account and make it as default account.
- Finally, restart Outlook with new profile.
Method 3: Rebuild Fresh File
If your OST file is connected correctly but still Outlook showing “cannot start Outlook OST is not an Outlook data file” error, then recreation technique could work.
Important Note: Only use this when your OST file doesn’t contain any offline data. Because after renaming and recreation makes your file becomes inaccessible.
- Close Outlook completely.
- Navigate to the OST file location.
- Rename existing OST file.
- Open Outlook.
- Allow Outlook to create a new OST file.
- Wait for mailbox synchronization process to complete.
And it will automatically download complete server mailbox data in a new OST file.
Professional Solution for Orphaned or Corrupt OST Files
As we suggest, use automated solution for damaged, orphaned, and corrupt OST files. Because no one can restore data from damaged OST file. It is a perfect software for users who are unable to access their offline Outlook files by moving them into PST file format. The application is so simple and easy to use which makes the entire process efficient for the users. This software can recover permanently deleted emails from OST including complete mailbox. Try the demo edition now:
How Does Solution Fix “Outlook OST is Not an Outlook Data File” Error?
- Download the tool on your system properly.
- Use “Add File” option to load affected data file.
- Select “Scan Modes” according to corruption level.
- Preview OST file data on the panel, and choose “Export” button.
- Select any saving option and use “Browse” button to select any location.
- Click on the “Export” button to start process.
Conclusion
We have resolved the “cannot start Outlook OST is not an Outlook data file” error by repairing damage profile, recreating a new file and using some basic techniques. If these techniques don’t work for you then go for the alternate solution.
Suggested tool helps users to recover data from corrupt OST file into healthy format including permanently deleted emails and other items. This is the only possible solution to import OST file into Outlook. Use demo version for better understanding.