Ordinarily, only hosts listed with RELAY_DOMAIN (Section 7.4.1.1) are allowed to relay through the local machine. This relay_entire_domain feature allows domains listed in the $=m class to also be relayed, including any hosts that end in any of the domains listed in the $=m class. This feature, how it is used, and how it fits into relaying and spam handling are explained in Section 7.4.5.