Let’s consider for a moment one way that email routing
might work. A user horatio in the domain
example.com has a workstation named
denmark. He could receive mail by using the
email address email@example.com. An
MTA with a message to deliver would simply look up the IP
address for denmark.example.com and deliver it to
that system for the user horatio. This scenario
requires that Horatio’s workstation is always turned on, that it has a
functional MTA running at all times to receive messages, and that it is
accessible by unknown MTAs from anywhere on the Internet. Rather than
manage hundreds or thousands of MTAs on workstations and expose them to
the Internet, nearly all sites make use of mail hubs that receive all
the mail for a domain. MTAs such as Postfix need a way to determine
which host or hosts are the mail hubs for a domain. DNS MX records provide this information.
A mail exchanger either delivers mail it receives or forwards it to another mail system. A domain may have multiple mail systems for reliability, and therefore multiple MX records. Generally, one host is the primary mail server and the others serve as backup or secondary mail servers. Each MX record in DNS contains a preference value that orders mail systems from most preferred to least preferred.
BIND is one of the most common DNS server applications. (O’Reilly’s DNS and BIND by Paul Albitz and Cricket Liu fully explains the DNS system and documents the BIND software.) A simple BIND ...