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16.3 A Limitation of MX Records
While we are on the topic of MX records,
let's talk about how they can result in mail taking
a longer path than necessary. The MX records are a list of data
returned when a name is looked up. The list isn't
ordered according to which mail exchanger is closest to the sender.
Here is an example of this problem: Your non-Internet-connected
network has two hosts capable of relaying Internet mail to your
network. One host is in the U.S., and one host is in France. Your
network is in Greece. Most of your mail comes from the U.S., so you
have someone maintain your zone and install two wildcard MX
recordsgiving the highest preference to the U.S. relay and a
lower preference to the France relay. Since the U.S. relay is at a
higher preference, all mail will go through that
relay (as long as it is reachable). If someone in France sends you a
letter, it will travel across the Atlantic to the U.S. mail relay and
back across the Atlantic from the U.S. mail relay to your network in
Greece because there is nothing in the MX list to indicate that the
French relay is closer to that sender.
