Sends the MDN receipt.
public func sendResponse() throws -> Void
By default the class will assume that your SMTP server is located at the same address as your incoming mail server. To set a different address, to use a non default port, see the various Config settings. To use SMTPS (SSL), set the various SSL properties.
A response will only be sent if requested by the client. If MDNTo is empty, this method will do nothing.