This policy setting controls whether Outlook will use remote certificate authority information in a secure email message ...

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.