SENDMAIL supports the concept of a mail "hub" that can be used for the local delivery of incoming mail. Any mail being received by the local mail server is immediately relayed to the mail server specified in the $H macro.

By using the $H macro, you can configure a departmental mail server to accept mail for a sub-domain or other network, while forcing delivery to be handled through a primary mail server for a domain.

Two other macros are used to define relay capabilities. The $R macro defines the out-bound relay host for any local recipients, while the $S macro defines an out-bound relay host for any non-local recipients.