User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other
User
HKCU\software\policies\microsoft\office\16.0\outlook\preferences
This policy setting controls whether users can rearrange their folders in Outlook. If you enable this policy setting, users cannot rearrange their folders. The folders are displayed alphabetically. If you disable this policy setting, users can rearrange their folders, but are not able to turn automatic alphabetical sorting back on from the Ribbon. If you do not configure this policy setting, users can rearrange their folders, and turn automatic alphabetical sorting back on from the Ribbon.