sendmail, 4th Edition

$S

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The smart host

mc configuration

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.