Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 236
Mail Exchange Records (MX)

One of the largest uses of the Internet is email, and DNS plays a major role in this as well. It does not send or receive mail, but it does provide information on the mail servers for a given name. In the database is a resource record of MX. DNS uses this single type of resource record for mail routing. It specifies a mail exchanger for a domain name. This mail exchanger is a host that will either process the mail or forward it on. Processing is simply the task of providing for delivery to the addressee or providing a path to another mail transport. It may also forward the mail to its final destination or pass it on to another mail exchanger in closer proximity of the recipient using SMTP (explained next). An example record is:

What the preceding record states is the mail exchanger for the domain engineering.naugle.com is mail.naugle.com. The number (5, in this case) is a precedence value. It can range from 0 to 65535. If there is only one MX for a domain, this field is useless. For example:

A mail program (such as SMTP) should use the mail exchanger with the lowest value first. If this fails, the next one associated with that domain is used. If there are no records associated with a domain or host, the mail must be able to deliver the message directly, and some versions of mail senders have this capability. The most common mail transport out there today is called sendmail.