Using the mc configuration method the $S macro stores the hostname defined by SMART_HOST (Section 4.3.3.6). The smart host is the name of the host that can deliver all mail that the local host cannot. $S is most often used with UUCP sites to get DNS mail to the outside world. Do not use this $S macro directly, as it might change without notice in a future version of sendmail. Instead, define this value using the mc SMART_HOST configuration macro.