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Why does Outlook state that the OST file cannot be opened?
The message “The file cannot be accessed” or “Outlook data file cannot be opened” may appear serious, but the reason is usually simple.
Corruption is the most frequent issue; Outlook will crash during the syncing process, your laptop may lose power, or the file may become too large for it to handle, causing the OST to fail. Profiles come next in frequency. Since an OST is associated with one account and one profile, if that profile is altered or damaged, Outlook no longer recognises the file.
The worst of the three reasons is that your Exchange or Microsoft 365 account has been deleted, transferred, or severed from its server and the OST therefore has nothing anymore to sync with.
Until you have worked out which case you are in, don’t begin deleting any files.
Fix 1: Start Outlook in Safe Mode
Safe Mode allows Outlook to be started without most of the add-ins, which is useful for carrying out a quick test. To do this, press Windows + R, type outlook.exe /safe and then press Enter.
If Outlook is opening then the issue is most likely due to an add-in; it is the old CRM connectors and the antivirus plugins that cause the most problems. You should go to File > Options > Add-ins, turn off the add-ins one by one and then restart Outlook after making each change. Once the error has disappeared you will have found the cause of the problem. Either update that add-in or remove it.
Fix 2: Run the Inbox Repair Tool
Microsoft provides ScanPST.exe for the repair of Outlook data files and you can usually locate it in C:\Program Files\Microsoft Office\root\Office16, although the folder name will differ depending on the version; however, it is quicker to search for the file name in Windows.
Save the Outlook file and copy the OST file to a safe location. Next, open ScanPST, go to the OST file, and carry out the scan. When errors are detected, select the Repair option.
Don’t place too many expectations on it since ScanPST was designed for PST files and usually doesn’t work when dealing with a severely damaged OST. The process only takes around ten minutes though, so it’s worth making one attempt.
Fix 3: Rename the OST and let Outlook rebuild it
It will only function if your mailbox still exists on Exchange or Microsoft 365; close Outlook and then open the folder C:\Users\YourName\AppData\Local\Microsoft\Outlook, rename the OST file in question to something such as old.ost and finally restart Outlook.
Outlook makes a new file and downloads your mail from the server; if you have a large mailbox and are using slow internet then it may take several hours, so do not stop it. You should keep the old file until you have checked that all your emails, folders, contacts and calendars have come back.
Note that if the account has been deleted or the server is no longer available, then this solution should be ignored since Outlook will have nothing to download and the old OST file will be the only one that contains that mail.
Fix 4: Create a new Outlook profile
In some cases the file is all right but the profile is the issue. You should open the Control Panel, search for Mail, click on Show Profiles and then select Add. Fill in the details for your account, make the new profile the default one and start Outlook.
You’ll receive a new OST; use it for a few days before getting rid of the old profile. Once the error has gone away, it means that the old profile was damaged.
Fix 5: Check your Outlook version and File Size
Go to File, then select Office Account, click on Update Options and choose Update Now. Older versions had bugs that Microsoft has already fixed. You should restart Outlook after the update is complete.
Make sure that you check the size of your OST file. The standard limit is around 50 GB, and a file at 48 or 49 GB may behave incorrectly or might fail to open. In that case, reduce the amount set aside for ‘Mail to keep offline’ or back up the old messages before carrying out any other actions.
What if all of the remedies fail?
The file cannot be loaded because it is too damaged or because the account associated with it no longer exists; this is a common situation following a move to a new Microsoft 365 tenant or after an old Exchange server has been retired. The employee still has the OST on their laptop, but no profile will allow it to be opened.
It won’t make any difference to the situation to create a new profile and in fact the problem gets worse if the file is deleted. What you need is a method of reading the data directly from the file already there.
Recover the data with BreviSoft OST to PST Converter
The BreviSoft OST to PST Converter is able to open OST files by itself, even if they are orphaned, inaccessible, or corrupted; in this case you do not need the original Exchange account.
To be able to preview the file before exporting anything, load it. If the file is in a good condition, you will be able to see the emails, folders, contacts, calendars, and attachments. You can save all the data as a PST file for Outlook or select only the folders that you need. You can divide a very large OST file into smaller PST files in order to avoid hitting the size limits once more. There are other export options which allow you to transfer the data into other email clients that are supported, so you are not obliged to use Outlook if you have switched.
If an IT administrator has to deal with an Exchange server that has been shut down, there are twenty staff laptops and twenty old OST files but no accounts to link them to. Rather than trying to access each mailbox, the administrator passes each file through the converter, checks the preview and saves the data in a usable format.
If you’re not sure about your file, try the trial version on one OST first. Make sure that your important folders and messages appear in the preview before processing the rest.
How to avoid OST errors in the future
Make sure that your mailbox doesn’t get out of hand. If your organization permits this, then you should archive your old mail into a PST once a year rather than allowing one OST to exceed 40 GB.
Look at Outlook the usual way; if you need to terminate it using Task Manager then it could lead to corruption, so only do this when the program is fully frozen. Make sure Office is up to date because the updates correct actual bugs.
Before you carry out any account migration or server shutdown, make sure that you export or convert your OST while the profile is still functioning. Simply carrying out that one habit will prevent the majority of the problems mentioned above. You should also periodically back up your Outlook data since a small copy can save you hours later.
Wrapping up
Begin with the simple solutions by switching to Safe Mode, checking your add-ins, running the repair tool, and creating a new profile if necessary. In the case where your mailbox is still on Microsoft 365 or Exchange, the problem is often solved by renaming the faulty OST file and allowing Outlook to rebuild it.
If the account or the server is deleted, rebuilding will have no effect since there is nothing to download.
If the file contains data you can’t lose, stop renaming and deleting it. Preview it with the BreviSoft OST to PST Converter and save OST as a PST or another supported format before you try anything else.
