|Hosts for whom to relay
||V8.9 and above
The class $=R holds as
its list of values the host and domain names that
sendmail should allow mail to be relayed to.
This $=R class should not be used directly because
it could change without notice in future versions of
sendmail. See RELAY_DOMAIN (Section 188.8.131.52) and RELAY_DOMAIN_FILE (Section 184.108.40.206).