User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange
User
HKCU\software\policies\microsoft\office\16.0\outlook
This policy setting controls whether departments can be picked as recipients in the Hierarchical Address Book (HAB). If you enable this policy setting, the tree control to pick departments as recipients is turned off in the HAB. If you disable or do not configure this policy setting, the tree control to pick departments as recipients is turned on in the HAB.