Outlook OST cannot be accessed, you must connect to Microsoft

Some Office users are unable to access Outlook on their computer. The error message pops up out of nowhere and they want to know how to resolve it. Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The file <filename>. Ost cannot be accessed. You must connect to Microsoft Exchange at least once before you can use your Outlook data file (.Ost).

Outlook OST cannot be accessed, you must connect to Microsoft Exchange

If you’re getting the error message “The file . Ost cannot be accessed. You must connect to Microsoft Exchange in Outlook,” try these solutions.

Edit Registry
Remove Outlook Credentials
Use Inbox Repair Tool
Repair Outlook

Outlook If you change the password for your Microsoft account, Outlook will ask you to reconnect to Microsoft Exchange to access your Outlook data file. However, you may not see the window where you need to enter your password. To resolve this issue, you will need to force Outlook to display the window where you enter your account name and password. To do this, launch the Registry Editor from the Start Menu or by pressing Win + R, type “Regedit” and hit Enter.

Once the Registry Editor window is launched, go to the following location.

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity

From the left panel, find Enable ADAL. If the value is not present, right-click on the empty space and select New > DWORD (32-bit) Value. Rename the newly created value to Enable ADAL. Now, double-click on it and set the Value Data to 0.

Now, look for DisableADALatopWAMOverride. If you can’t find it, create a new value and name it DisableADALatopWAMOverride. Set the Value Data of this value to 1. Close the Registry Editor and restart your computer. Launch Outlook and see if it worked.

Remove Outlook Credentials

Next, try to remove your credentials using Windows Credential Manager and then add them again to see if that helps. To do this, launch Credential Manager from the Start Menu and then the credentials for Outlook. Once the credentials are removed, launch Outlook and sign in again.

Launch Credential Manager from the Start Menu.
Look for Outlook credentials, it will have the keyword Outlook.
Expand it and click on the Remove button.

Once done, launch Outlook, and sign in. Hopefully, this will do the job for you.

Why is my Outlook not connecting to the Exchange Server?

Your Outlook may not be able to connect to the Exchange Server if your bandwidth is low. You could try to fix this by checking your Internet Speed with a free Speed Tester. In the case that your Internet connection is not the problem, something might be wrong with your Outlook Profile. We would recommend checking this post on what to do when the connection to Microsoft Exchange Server is unavailable. Hopefully, you will be able to resolve the issue easily.

How do I connect my Microsoft Exchange account?

In order to connect to your client’s Microsoft Exchange account, the administrator will need to enable the feature from the Manage application > Extension tab. Once that is done, you can follow the prescribed steps to connect to the Microsoft Exchange account.

Go to Settings > Extension.
Look for the Microsoft Exchange extension and click Connect.
Select the Authentication Method and follow the on-screen instructions to connect to your account.