@Pitmaster, what you want is a multi-tenant mail environment. Unfortunately NethServer does not provide that.
Webtop5 has the option to act as multi-tenant mail solution, but that feature is not implemented in NethServer.
@gabriele_bulfon has explained it in this post
For the rest I refer to what @saitobenkei said: you can create domain users with the domain you configured when you installed your Samba4 domain or LDAP domain.
You can add several other mail domains so your domain users can send and receive with alias email addresses. Those aliases can be either created from the initial domain or with another mail domain.