A mail relay is a system that resends mail that it receives. When mail that should be delivered by some other host arrives at the mail relay host, the system decides whether it should relay the mail. If relaying is allowed, the relay host sends the mail on to the destination address. If relaying is denied, a “Relaying denied” error message is returned to the sender. This chapter contains recipes that control when relaying is allowed or denied and recipes to configure a system to make use of a mail relay.
Relaying is different from forwarding. Mail that is forwarded arrives at the system addressed to the local host; it is forwarded only if the host is instructed to do so by the aliases database or the .forward file. Mail that is relayed arrives at the system addressed to some other host; it is only relayed if sendmail is configured to allow relaying.
In the same way that sendmail must be configured to act as a mail relay, a system must be configured to use a mail relay. Any system running sendmail can directly deliver its own mail; sendmail does not depend on relays by default. However, there are a variety of different sendmail configurations that use relay servers:
A system can be configured to send all of its mail to a relay. This type of configuration is called a null client , not because the sendmail.cf file is empty, but because the system depends on an external host for all email service.
A system can be configured ...