It depends on the permissions you delegated to this client.
- Create the client
- If you don't want him to manage his domain, while creating the client in the "Limits" tab put 0 to all domain-related fields. In this case you should later add his domain manually from administrator's account.
- Set necessary quota and other mail-related params (amount of allowed mailboxes, aliases, routes, etc).
- Add domain and email domain for this client from administrator's account.
- Send to client his credentials and he'll be able to manage his mailboxes, but not allowed to delete or change his domain.