SENDMAIL supports the concept of a local "relay" server that can be used to process with a recipient address that does not contain an "@domain" qualifier.

For example, if user Mary were to send mail to "Joe", and the $R macro were defined, then the local mail server would relay the mail to the SMTP server specified by the $R macro. The remote server would then process the mail, either delivering it to Joe’s mailbox, or forwarding to Joe’s mail server for delivery.

This macro can be used to build a multi-server installation, although this is not the preferred method. Instead, Unoverica Corporation recommends that users create a multi-tiered alias system as documented in section 5.4 Scenario 3: Multi-Server Installations.

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 $S macro defines an out-bound relay host for any non-local recipients.