Forwarder stations are not necessarily "smarter" about mail routing than other stations in the network, but they are "better connected." Forwarder stations deliver mail to "all points beyond" some point in the domain name space.
The designation of the forwarder station is primarily determined by the physical topology of the network; the default sendmail.cf file can designate only a single forwarder, and the name of that station must be hard-coded in the configuration file.
The following parameters designate a mail forwarder in the /usr/etc/configmail script and the /etc/sendmail.cf file: