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One of the advantages of the Domain Name System over host tables is its
support of advanced mail routing. DNS offers a mechanism for specifying backup
hosts for mail delivery. The mechanism also allows hosts to assume mail
handling responsibilities for other hosts. This lets diskless workstations
that don't run mailers, for example, have mail addressed to them processed by
their server. These features give administrators more flexibility in
configuring electronic mail on their network.
MX Records
DNS uses a single type of resource record to implement enhanced mail routing,
the MX record. MX records specify a mail exchanger for a domain name, a host
that will either process or forward mail for the domain name.
In order to prevent mail routing loops, the MX record has an extra parameter,
besides the domain name of the mail exchanger, a preference value like:
peets.mpk.ca.us IN MX 10 relay.hp.com specifies that
relay.hp.com is a mail exchanger for
peets.mpk.ca.us at preference value 10.
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