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11.5. Mail Routing on the Internet
On the Internet, the destination
host's configuration determines whether any specific
mail routing is performed. The default is to deliver the message to
the destination by first determining what host the message should be
sent to and then delivering it directly to that host. Most Internet
sites want to direct all inbound mail to a highly available mail
server that is capable of handling all this traffic and have it
distribute the mail locally. To announce this service, the site
publishes a so-called MX record for its local domain in its DNS
database. MX stands for Mail
Exchanger and basically states that the server
host is willing to act as a mail forwarder for all mail addresses in
the domain. MX records can also be used to handle traffic for hosts
that are not connected to the Internet themselves. These hosts must
have their mail passed through a gateway. This concept is discussed
in greater detail in Chapter 6.
MX records are always assigned a
preference. This is a positive integer. If
several mail exchangers exist for one host, the mail transport agent
will try to transfer the message to the exchanger with the lowest
preference value, and only if this fails will it try a host with a
higher value. If the local host is itself a mail exchanger for the
destination address, it is allowed to forward messages only to MX
hosts with a lower preference than its own; this is a safe way of
avoiding mail loops. If there is no MX record for a domain, or no MX
records left that are suitable, the mail transport agent is permitted
to see if the domain has an IP address associated with it and attempt
delivery directly to that host.Suppose that an organization, say Foobar, Inc., wants all its mail
handled by its machine mailhub. It
will then have MX records like this in the DNS database:
green.foobar.com. IN MX 5 mailhub.foobar.com.This announces mailhub.foobar.com as
a mail exchanger for green.foobar.com with a preference of 5. A
host that wishes to deliver a message to
This is a very
simple description of how MX records work. For more information on
mail routing on the Internet, refer to RFC 821, RFC 974, and RFC
1123.
11.5. Mail Routing on the Internet
On the Internet, the destination
host's configuration determines whether any specific
mail routing is performed. The default is to deliver the message to
the destination by first determining what host the message should be
sent to and then delivering it directly to that host. Most Internet
sites want to direct all inbound mail to a highly available mail
server that is capable of handling all this traffic and have it
distribute the mail locally. To announce this service, the site
publishes a so-called MX record for its local domain in its DNS
database. MX stands for Mail
Exchanger and basically states that the server
host is willing to act as a mail forwarder for all mail addresses in
the domain. MX records can also be used to handle traffic for hosts
that are not connected to the Internet themselves. These hosts must
have their mail passed through a gateway. This concept is discussed
in greater detail in Chapter 6.
MX records are always assigned a
preference. This is a positive integer. If
several mail exchangers exist for one host, the mail transport agent
will try to transfer the message to the exchanger with the lowest
preference value, and only if this fails will it try a host with a
higher value. If the local host is itself a mail exchanger for the
destination address, it is allowed to forward messages only to MX
hosts with a lower preference than its own; this is a safe way of
avoiding mail loops. If there is no MX record for a domain, or no MX
records left that are suitable, the mail transport agent is permitted
to see if the domain has an IP address associated with it and attempt
delivery directly to that host.Suppose that an organization, say Foobar, Inc., wants all its mail
handled by its machine mailhub. It
will then have MX records like this in the DNS database:
green.foobar.com. IN MX 5 mailhub.foobar.com.This announces mailhub.foobar.com as
a mail exchanger for green.foobar.com with a preference of 5. A
host that wishes to deliver a message to
This is a very
simple description of how MX records work. For more information on
mail routing on the Internet, refer to RFC 821, RFC 974, and RFC
1123.
