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DNS on Windows 2000, Second Edition by Cricket Liu, Matt Larson

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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 is not 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—with 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. 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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