1 - Close Outlook completely. If you do not do this you will not be able to remove offline cached files

2 - Click Start then Control Panel


3 - Search for Mail


4 - Click on Email Accounts…


5 - Go to Data Files


6 - Click Open File Location. After you open, go back to the E-mail tab


7 - Keep this Window open as we need to come back to it later on Step 12


8 - Click Change


9 - You will see Server, a tick for use Cached Exchange Mode and User Name


10 - Untick Use Cached Exchange Mode, then click Next


11 - Then if all OK click Finish




12 - Return to the .ost file location from Step 7. You can now delete the .ost file


13 - Now open Outlook. If all gone well you should have no errors or prompts