KnownBooleanMetadataFieldsUntrusted Field

Outlook email object associated metadata field. True if the incoming message arrived over an external link other than X.400 or the Internet. It originated either outside the organization or from a source the gateway does not consider trusted (MAPI property PidTagMessageFlags 0x00008000 bit).

[KnownMetadataAttribute(IsEmailType = true)]
public const string Untrusted = "Untrusted"