Domain Mapping and Independant Users and domains

Hello

I'd like setup network with totaly independant websites and domains. I think have domain1.com and mapped domain2.com on subdomain.domain1.com

Thus having no interraction at all between users of different domains.

I'm confuse about all this.
How can I do that ?
Because I've tried it and when I register a test username on domain2com, a message is sended from domain1.com; not from domain2.com

I forgot something ?
Thanks for help