The PROPID_M_SIGNATURE_LEN property indicates the length of the message's digital signature.
This property is used for messages sent with a digital signature to an MSMQ Connector.
When passing PROPID_M_SIGNATURE_LEN to MQReceiveMessage, the corresponding VT field in the aPropVar array can be set to VT_NULL.
MQReceiveMessage, PROPID_M_SIGNATURE