Believe this is set on the policies for OWA. Navigate to the Vault admin console and select polices and mailbox polices. Open the propereties of this and select advacned tab. From select list choose OWA and then review the polices that can be set.
Once you have set this you with either have to force or wait for a sync ot take place.
On your PC you are using for testing delete the tempr internet files from IE and restest.