ActiveX One-Off forms must be configured.

By default, third-party ActiveX controls are not allowed to run in one-off forms in Outlook. You can change this behavior so that Safe Controls (Microsoft Forms 2.0 controls and the Outlook Recipient and Body controls) are allowed in one-off forms, or so that all ActiveX controls are allowed to run.

Verify the policy value for User Configuration -> Administrative Templates -> Microsoft Outlook 2016 -> Security "Allow Active X One Off Forms" is set to "Enabled: Load only Outlook Controls".

Procedure: Use the Windows Registry Editor to navigate to the following key:

HKCU\Software\Policies\Microsoft\Office\16.0\outlook\security

Criteria: If the value AllowActiveXOneOffForms is REG_DWORD = 0, this is not a finding.

Set the policy value for User Configuration -> Administrative Templates -> Microsoft Outlook 2016 -> Security "Allow Active X One Off Forms" to "Enabled: Load only Outlook Controls".