188.8.131.52.2.2 Recognizing a Message Object that Represents an Opaque-Signed or Encrypted S/MIME Message
If a Message object has the message class (PidTagMessageClass property ([MS-OXCMSG] section 184.108.40.206)) value of "IPM.Note.SMIME" and contains exactly one Attachment object, it SHOULD be treated as an opaque-signed message or an encrypted 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) is either "application/pkcs7-mime" or "application/x-pkcs7-mime", or it is "application/octet-stream" and filename, as specified by the PidTagAttachFilename property ([MS-OXPROPS] section 2.584), and has a file extension ".p7m") and represents a valid encrypted or opaque-signed message, as specified in [RFC3852]. If the value of the message class is not "IPM.Note.SMIME", but ends with the suffix ".SMIME", then the Message object MAY<11> be treated as an opaque-signed message or an encrypted message.
The message class value "IPM.Note.SMIME" can be ambiguous.<12>
If a Message object has a message class value of "IPM.Note.SMIME" does not have the appropriate structure or content as specified in section 220.127.116.11, then the behavior is undefined.