A URL indicating how the receipt is to be delivered.
The default mode of operation is for the receipt to be returned synchronously within the HTTP reply. By specifying a valid URL, the user may request asynchronous delivery instead. The URL indicates the destination for the reply, and may use any appropriate protocol, such as "http", "https", or "mailto".
If MDNDeliveryOption is set to an empty string, the receipt will be returned synchronously, and will be processed automatically by the class. Clients requesting asynchronous delivery should provide their own processing for reading receipts.