User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange
User
HKCU\software\policies\microsoft\office\16.0\outlook\autodiscover\redirectservers
This policy setting allows you to specify HTTPS server names for which Autodiscover won’t show a warning message when redirecting from HTTP to HTTPS. By default, when an Autodiscover operation redirects from HTTP to HTTPS, you may be shown a warning message about the redirection. If you enable this policy setting, you need to specify HTTPS server names, and for those server names, you won’t be shown a warning message. For example, if you enter contoso.com, you won’t be shown a warning message when Autodiscover redirects to https://contoso.com. If you disable or don’t configure this policy setting, you may be shown a warning message when an Autodiscover operation redirects from HTTP to HTTPS.