The textual message that is included in the receipt.
The human-readable portion of the receipt generated in MDN.
In the Send Adapter, this property will be populated after the full receipt is received from the server, if synchronous MDN delivery was requested.
In the Receive Adapter, if ReceiptMessage is not set, an appropriate message will automatically be generated and added to the receipt. The message will differ depending on whether the message was processed successfully, whether the sender's signature was authenticated, etc. If set, the value of ReceiptMessage will override what is generated by the adapter. If set to ".", a blank message will be generated.