Introduce Service Interface with requirement to send, receive and create Zend\Mail\Messages

Zend\Mail\Message should be the base requirement for Email Service Interface methods sendMessage & receiveMessage. If a Zend\Mail\Transport is used, a Zend\Mail\Message will suffice. For ZF2x transports, ZF2x\Mail\Message is preferable as potentially less complicated. 

Email Service must support sending ZF2x\Mail\Messages via a Zend\Mail\TransportInterface which is handled by ZF2x\Mail\Message::prepare()

Email Service must support sending Zend\Mail\Messages via ZF2x\Mail\TransportInterface. A ZF2x\Mail\Message conversion implementation is required: ZF2x\Mail\Service::baseMessageToZF2xMessage(ZendMailMessage $message)

Fixed issue with reply-to header handling.