Using the mc
configuration technique, the $R macro stores the
hostname defined by LOCAL_RELAY (Section 4.5.4). If
$H has a value and if $R does
not, all local email is forwarded to the hub
defined for $H. If $R is
defined, it takes precedence over $H for some
mail. See Section 4.5.7 for a description of MAIL_HUB
and how it interacts with LOCAL_RELAY.
Note that you should not define this $R macro
directly because later versions of sendmail
might use a different macro. Instead, use the LOCAL_RELAY
mc macro for this purpose.