User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security > Cryptography > Signature Status dialog box
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting controls how Outlook functions when a root certificate is missing. If you enable this policy setting, you can choose from three options that determine how Outlook functions when a root certificate is missing. - Neither Error nor Warning. This option displays neither an error nor a warning, and enforces the default configuration in Outlook. - Warning. This option ensures that Outlook displays a warning message when a root certificate is missing. - Error. This option ensures that Outlook displays an error message when a root certificate is missing. If you don't configure this policy setting, users will see an error when a root certificate is missing.