User Configuration > Administrative Templates > Microsoft Outlook 2016 > Security > Cryptography
User
HKCU\software\policies\microsoft\office\16.0\outlook\security
This policy setting controls whether Outlook will use remote certificate authority information in a secure email message to validate that its certificate is trusted. If you enable this setting, you’ll allow the operating system to access remote network locations specified in a certificate for validation. If you disable or don’t configure this setting, retrieval of remote Certificate Authority Information won’t be allowed, and only stored certificates will be used for authentication.