Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 517)

DNS Service 517
If you want to change your mail server or redirect mail, you have to notify potential senders
of a new address for your users. Or, you can create an MX record for each domain that you
want handled by your mail server and direct the mail to the correct computer.
When you set up an MX record, you should include a list of all possible computers that can
receive mail for a domain. That way, if your server is busy or down, mail is sent to another
computer. Each computer on the list is assigned a priority number. The one with the lowest
number is tried first. If that computer isn’t available, the computer with the next lowest
number is tried, and so on. When a computer is available, it holds the mail and sends it to
the main mail server when the main server becomes available, and then the server delivers
the mail. A sample list might look like this:
example.com
10 reliable.example.com
20 our-backup.example.com
30 last-resort.example.com
MX records are used for outgoing mail, too. When your mail server sends mail, it looks at the
MX records to see whether the destination is local or somewhere else on the Internet. Then
the same process happens, in reverse. If the main server at the destination is not available,
your mail server tries every available computer on that destination’s MX record list, until it
finds one that will accept the mail.
If you don’t enter the MX information into your DNS server correctly, mail won’t work. For
more information about MX records, see the resources listed at the end of this chapter.
Setting Up DNS Service for the First Time
If you are using an external DNS name server and you entered its IP address in the Setup
Assistant, you don’t need to do anything else. If you are setting up your own DNS server,
follow the steps in this section.
Step 1: Register your domain name
Domain name registration is managed by a central organization, the Internet Assigned
Numbers Authority (IANA). IANA registration makes sure domain names are unique across
the Internet. (See www.iana.org for more information.) If you don’t register your domain
name, your network won’t be able to communicate over the Internet.
Once you register a domain name, you can create subdomains within it as long as you set up
a DNS server on your network to keep track of the subdomain names and IP addresses.