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MX, Mail Exchange Records

MX records are far more important than they sound. They allow all mail for a domain to be routed to one host. This is exceedingly useful -- it abates the load on your internal hosts since they do not have to route incoming mail, and it allows your mail to be sent to any address in your domain even if that particular address does not have a computer associated with it. For example, we have a mail server running on the fictitious machine eric.foobarbaz.com. For convenience sake, however, we want our email address to be user@foobarbaz.com rather than user@eric.foobarbaz.com. This is accomplished by the record shown below:

   foobarbaz.com. IN MX 10 eric.foobarbaz.com.

The column on the far left signifies the address that you want to use as an Internet email address. The next two entries have been explained thoroughly in previous records. The next column, the number 10, is different from the normal DNS record format. It is a signifier of priority. Often larger systems will have backup mail servers, perhaps more than one. Obviously, you will only want the backups receiving mail if something goes wrong with the primary mail server. You can indicate this with your MX records. A lower number in an MX record means a higher priority, and mail will be sent to the server with the lowest number. If something happens so that this server becomes unreachable, the computer delivering the mail will attempt every other server listed in the DNS tables, in order of priority. Obviously, you can have as many MX records as you would like. It is also a good idea to include an MX record even if you are having mail sent directly to a machine with an A record. Some sendmail programs only look for MX records. It is also possible to include wildcards in MX records. If you have a domain where your users each have their own machine running mail clients on them, mail could be sent directly to each machine. Rather than clutter your DNS entry, you can add an MX record like this one:

   *.foobarbaz.com. IN MX 10 eric.foobarbaz.com.

This would make any mail set to any individual workstation in the foobarbaz.com domain go through the server eric.foobarbaz.com. One should use caution with wildcards; specific records will be given precedence over ones containing wildcards.

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Related information

CNAME Records | Email services | POP3 accounts

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