You can delete the domain using the
button. A domain cannot be deleted if:user accounts or groups have been already defined within the domain. All accounts must be deleted first (for details, see chapter 8.5 Removing user accounts).
the aliases are defined in it. First, delete all the aliases (for details, see chapter 12.3 Aliases).
it is the primary domain. However, you can create another domain and define it as primary. Then, the former domain can be deleted.