This message is usually displayed because you have added a resource email account with user credentials rather than using the approved method of delegate access to access the inbox of the account. The full version of this message typically reads:“Cannot expand the folder. The set of the folders cannot be opened. The file C:\Users\*Username*\AppData\Local\Microsoft\Outlook\*EmailAddress*.ost is not an Outlook data file (.ost).” Where *Username* is your username and *EmailAddress* is the resource account that you are trying to access.
To resolve this issue you will need to remove the locally added resource email accounts.
- In Outlook, click File.
- Then Account Settings and Account Settings.
- Then select the resource email account and then select Remove.
- Click Yes on the pop-up.
- Close and re-launch Outlook.
Accessing Resource Email accounts
To restore access to the resource email account please ensure you have completed both sections of the Managing another user’s mailbox or generic mailbox in Outlook article.