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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 the domain name of a mail destination is looked up. The list isn't ordered according to which 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 records—the highest preference to the U.S. relay and a lower preference to the relay in France. 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. and back because there is nothing in the MX list to indicate that the French relay is closer to that sender.

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