User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous > PST Settings
User
HKCU\software\policies\microsoft\office\16.0\outlook\setup
This policy setting allows you to specify that only the default data file is upgraded on the first boot of Outlook. If you enable this policy setting, only the data file associated with your delivery mailbox is upgraded. If you disable or do not configure this policy setting, all Outlook Data Files are upgraded.