Attachments opened from Outlook must be in Protected View.
This policy setting allows for determining if PowerPoint files in Outlook attachments open in Protected View. If enabling this policy setting, Outlook attachments do not open in Protected View. If disabling or not configuring this policy setting, Outlook attachments open in Protected View.
Set the policy value for User Configuration -> Administrative Templates -> Microsoft PowerPoint 2016 -> PowerPoint Options -> Security -> Trust Center -> Protected View "Turn off Protected View for attachments opened from Outlook" to "Disabled".