The $S macro is used to define a "smart" relay host that will process and deliver all non-local mail on your behalf. This may be required if you use a firewall or an ISP’s mail server to deliver mail to external networks.

Any mail being sent to a non-local recipient (as determined by the recipient’s e-mail address containing a non-local domain name) will be forwarded to the host specified by the $S macro.

For more information on how to configure Unoverica Message Transport to work with an external network or server, refer to section 4.3 Routing Mail Through an External Site.

Two other macros are used to define relay capabilities. The $H macro defines an incoming mail hub that inbound mail is sent to, while the $R macro defines a local relay hub that is used when out-bound mail is being sent to a local user.