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.