Go to Control Panel, Mail (Outlook 2016) to get the below box appear.
Click on Email Accounts.
Highlight users email address and then click on Change.
Click on More Settings.
Click on Outlook Data File Settings.
Click on the Compact button. Then OK OK until compacting has completed.
Now go back into Mail (Outlook 2016).
Untick the Download Share Folders box. Apply and OK to complete. Come out of Mail completely and restart Outlook.