I am with Thomas on this one. It is really annoying to have to:
1.) take the time to set up a email account that will never be used
2.) use part of the license that will never be used.
And to call it essential for managing permissions, just outlines a problem in code. Let's consider unix for example: manages permissions where you can have user specific, but not give permissions to a group. Or vice-versa. Smartermail can have three groups, SU, Domain Admin, and User. SU has access to everything, why can't we create a single user under a domain just in the "User" group? The domain is always manageable by SU even without impersonating the domain admin user, making the domain admin pointless. Code-wise, I can understand the concern of a domain created without anyone to manage it. It becomes an orphan. But that is never the case, as SU is always present. You just need a checkbox when you create the domain, "make this user a domain admin". Else, the default, is to just make a regular user. And if a domain requires one user else it breaks things, just need a test for deleting the last user. This seems relatively trivial to implement.