The PROPID_M_EXTENSION property provides a place to put additional information that is associated with the message.
The PROPID_M_EXTENSION property is typically used by applications sending messages to or reading messages from a foreign queue. It is the application's responsibility to understand the content of this property.
The receiving application can determine the length of the information in this property by calling PROPID_M_EXTENSION_LEN.
Windows NT: Requires version 4.0 SP3 or later.
Windows: Requires Windows 95 or later.
Windows CE: Unsupported.
Header: Declared in mq.h.
Import Library: Use mqrt.lib.
Unicode: Defined only as Unicode.