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 turn off the "Download Headers and then Full Items" option. Microsoft Exchange Server 2003 or later is required. If you enable this policy setting, you will turn off the "Download Headers and then Full Items" option in the Download Preferences menu in the Send/Receive tab. If you disable or do not configure this policy setting, you will allow the "Download Headers and then Full Items" option in the Download Preferences menu in the Send/Receive tab.