An improvement included in the November 1 2016 update for Outlook 2016 (and in a previous update for Outlook 2013) is the ability to change the 'Warn me before downloading content when editing, forwarding, or replying to e-mail' option by setting a registry key. With this key set, the option to warn on replies and forwards in Trust Center, Automatic Downloads, is grayed out and can't be changed from the UI. This also removes the check box from the warning dialog so end-users can't disable the warning.
If the option to warn before downloading content when forwarding or replying is unchecked when this key is set, the user will not be warned. If the option is checked (the default value) and the new registry key set, the user will be warned and will not have the option to disable the warning.
To use this key, open the registry editor and browse to the Outlook Preferences key for your version of Outlook and add a new DWORD.
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences\ DWORD: ForceWarnAboutExternalContent Value: 1
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences\ DWORD: ForceWarnAboutExternalContent Value: 1
Do It For Me
Office 365 subscribers should have the update installed, volume licenses will need the appropriate update installed:
October 4, 2016, update for Outlook 2013 (KB3118367)
November 1, 2016, update for Outlook 2016 (KB3127912)