Used to indicate that a message disposition notification is requested.
If this property is set, a Disposition-Notification-To header will be added to the request, and an MDN will be requested. The value may be an email address, URL, etc., and while its presence is used to determine whether or not an MDN is sent, the value itself will typically be ignored by the server.
By default, the class will request a PKCS#7 signature and synchronous delivery. You may set MDNDeliveryOption to request an asynchronous MDN, and you may set MDNOptions to request a different type of signature, or no signature at all.