Prevent that domains with server_id 0 can be added
Prevent that domains with server_id 0 get added. This can happen when mail services are disabled under system > server services for all servers and then a mail domain is added. This domain gets server_id 0 and will therefore not work at all as it is not located on any server and sub-items like mailboxes for this mail domain will not work as well. The same might be possible for websites and dns zones as well when the web/dns services are disabled.