I might be missing something here but the domain name is what you
buy/what you get.
Note I own (techically it's more of a rental or lease) multiple domains hosted across multiple servers which I also fully control (no sharing, full root control).
As I understand it, a mail server has to use smtp to physically transfer mail between physical hosts and that requires one name that will resolve to an IP. Even if the server is hosting multiple domains, the mail for each still has to use the one "mailname" for transport.
Hence my original question.
-Tom