184.108.40.206.3 Recognizing a Message Object that Represents a Clear-Signed Message
If a Message object has a message class (PidTagMessageClass property ([MS-OXCMSG] section 220.127.116.11)) value of "IPM.Note.SMIME.MultipartSigned" and contains exactly one Attachment object, it SHOULD be treated as a clear-signed message. Additional verification steps can be performed to verify that the Attachment object is marked with the appropriate media type (for example, the PidTagAttachMimeTag property ([MS-OXPROPS] section 2.593) has a value of "multipart/signed") and represents a valid multipart/signed MIME entity as specified in [RFC1847]. If the message class value is not "IPM.Note.SMIME.MultipartSigned" but it ends with the suffix ".SMIME.MultipartSigned", the Message object MAY<7><8> be treated as a clear-signed message.
If a Message object with a message class value of "IPM.Note.SMIME.MultipartSigned" does not have the structure specified in section 18.104.22.168, the behavior is undefined.