Used to indicate the options requested for the MDN receipt.
"signed-receipt-protocol=optional, pkcs7-signature; signed-receipt-micalg=optional, sha-256"
By default, the class will request that the MDN be signed with a PKCS#7 signature over a SHA-256 hash, which is the industry standard.
Set MDNOptions to an empty string to request an unsigned receipt.
This property will automatically be updated when SignatureAlgorithm is set. Normally you will not need to set this property, however you can set a value here to override the automatically generated value.
The string format is that of the Disposition-Notification-Options HTTP header, as specified in RFC 3335. As a form of shorthand, you may set this property to "sha1", "sha-256", or "md5" to request the indicated hash algorithm.