The PROPID_M_MSGID property indicates the MSMQ-generated identifier of the message.
MSMQ generates a 20-byte message identifier and attaches it to the message when the message is sent. The identifier is an array of bytes that can be read by either the sending or receiving application.
MSMQ generates message identifiers for all messages, including acknowledgment messages generated by MSMQ and MSMQ connector applications. When an acknowledgment message is created, the identifier of the original message can be found in the acknowledgment message's PROPID_M_CORRELATIONID property.
To read a message's identifier, pass PROPID_M_MSGID to MQSendMessage when sending the message, or to MQReceiveMessage, when reading the message in the queue.
MQReceiveMessage, MQSendMessage, PROPID_M_CORRELATIONID