User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Delegates
User
HKCU\software\policies\microsoft\office\16.0\outlook\cached mode
This policy setting allows you to control the caching of shared mail folders. If you enable this policy setting, Outlook will only cache shared non-mail folders. If you disable or do not configure this policy setting, shared mail and non-mail folders you have access to are cached in your .ost file when you add another mailbox to your profile.