Ordinarily, the names listed with RELAY_DOMAIN (Section 18.104.22.168), those allowed to relay through the local
machine, are names of domains. By declaring this
relay_hosts_only feature, you cause the names in
that list to be interpreted as the names of hosts, not domains. This
feature, how it is used, and how it fits into relaying and spam
handling are explained in Section 7.4.6.