The PROPID_M_CONNECTOR_TYPE property indicates that some message properties typically generated by MSMQ are generated externally from MSMQ. These properties include several security and acknowledgment properties.
This property is typically required whenever a message is passed by a connector application. The connector type is required so that the sending and receiving applications know how to interpret the security and acknowledgment properties of the messages.
For more information, see MSMQ Connector Server.