Relay stations are not necessarily "better connected" than other stations in the network, but they are "smarter" about mail routing. They deliver mail to "all points within" some point in the domain name space.
For example, a company with a domain company.com has configured sendmail to treat alpha.company.com as the forwarder station and omega.company.com as the relay station. sendmail assumes that alpha.company.com is ultimately responsible for all mail to domains other than company.com and that station omega.company.com is ultimately responsible for all mail to the company.com domain itself. Note that there is nothing to prevent the relay and forwarder functions from residing on the same station.The designation of a relay station is primarily determined by administrative decision. sendmail can recognize a number of relay stations.
The relay station name is a special name used to identify relay stations in the network. This special name is defined by means of the R macro and is typically the name "relay." A relay station is so designated by being aliased to the name "relay." The default sendmail.cf file probes for a station named or aliased to the special relay station name and delivers mail to any such station in preference to the actual destination station. Mail is also sent to relay stations whenever the local station cannot determine the proper routing.
The following parameters designate a mail relay in the /usr/etc/configmail script and the /etc/sendmail.cf file: