So I have a client that has their active directory domain name (A-Example.com) for their organization. They acquired the new domain name (Z-Example.com) that they want to integrate into their existing AD infrastructure. They don't mind using (A-Example.com) for a while as user login. They want to be able to receive e-mail with either (tom@A-Example.com or tom@Z-Example.com) domain but send e-mail with the new domain (tom@Z-Example.com).
The reasoning behind this is because A-Example.com used to be the company and the brand now the company wants to differentiate itself (as Z-Example.com) from the brand (A-Example.com) and maybe add other brands down the road.
What would be the easier and more cost effective way of achieving the objectives above.