User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange > Cached Exchange Mode
User
HKCU\software\policies\microsoft\office\16.0\outlook\cached mode
By default, when users click Send/Receive or press F9 for Cached Exchange Mode accounts, Outlook synchronizes with the Exchange server. When this setting is enabled, clicking Send/Receive and pressing F9 do not synchronize with Exchange unless only one folder is being synchronized. Users can continue to use shift-F9 to synchronize the current folder.