User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange > Cached Exchange Mode
User
HKCU\software\policies\microsoft\office\16.0\outlook\cached mode
This policy setting allows you to force Outlook to use the Online Global Address List for ambiguous name resolution when composing messages in Outlook, instead of using the Offline Address Book when it is available. If you enable this policy setting, addresses are resolved using the Online Global Address List, which may contain additional information (that the Offline Address Book would not have) that allows an address to be resolved. If you disable or do not configure this policy setting, Outlook resolves addresses using the Offline Address Book when it is available.