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 firstname.lastname@example.org rather than
email@example.com. This is accomplished by the record shown
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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