I am a bit confused now with what you are trying to do, but you can have multiple domains under one install, with each domain pointing to the same install, as parent portals, on the same IP address. As Rob stated, emails are setup separately, outside of DNN. Email addresses are then applied to user ids in DNN, with the exception of one default email address for the host settings page.
There is only one DNN "host" per install, since the host account manages the entire install. If you want clients to have full host level access, go with separate installs for each client. They can be on the same server, and same IP address, but just separate websites in IIS and separate folders & database for the installs.
If you are trying to place multiple client domains under one install and host it for the clients, you can setup a parent portal for each client, with their domain, and assign an email address under the clients domin to the admin account for that portal when creating it. Emails sent from the client sites would use the email address applied to the admin account, and they would not see the host info at all. It would look like a completely separate site.
If adding new sites under separate domains in a shared install, set DNS to point the domain to the server. In IIS, for the website over the install, add the domain name to host headers, and then login as host on the portal and create a new portal as a parent, wth the clients domain name. You then set the admin email address to one for the same domain at that time. In this scenario, you would not give clients host accounts, since this would give them access to all portals in the install.
I am not sure this is what you are looking for, but hopefully it helps.