Received Mail: MX Records

A mail address consists of a username and a host address. The host address takes the form of a fully qualified domain name, listing the hostname and the domain name, separated by periods. Most uses of a hostname, such as FTP connections, translate the hostname into an IP address and use the IP address to locate the host system. Mail messages operate nearly the same way. However, they make use of the Domain Name Service to determine which host to actually send a message to. The host specified in the mail address may not be the host to which delivery should actually be made. Different networks will often specify a mail server to which mail for the hosts in a network should be delivered. For example, mail addressed to the rabbit.mytrek.com host may actually be delivered to the turtle.mytrek.com host. turtle .mytrek.com may be running a POP mail server that users on rabbit.mytrek.com can access to read their mail.

Such mail servers are associated with different hosts by mail exchange records, known as MX records, in a network's DNS configuration (see Chapter 34). When mail is received in a network, the network's DNS configuration is first checked for MX records to determine if the mail is to be delivered to a host different from that in the mail message address. For example, the following MX record says that any mail for the rabbit.mytrek.com host is to be delivered to the turtle.mytrek.com host. turtle.mytrek.com is the mail exchanger for rabbit.mytrek.com:

rabbit.mytrek.com. IN   MX    0   turtle.mytrek.com.

A host could have several mail exchangers, each with a different priority. If one is down, the one with next highest priority will be accessed. Such a design provides for more robust mail delivery, letting a few well-maintained servers handle received mail, instead of each host on its own.

Mail exchange records are also used for mail addresses for which there are no hosts. For example, you could designate virtual hosts or use the domain name as an address. To use a domain name, you would have an MX record with the domain name mapped to a mail server on the network. Mail addressed to the domain name would be sent to the mail server. For example, with the following MX record, mail sent to mytrek.com would be delivered to turtle.mytrek.com, which would be running a mail server like Sendmail:

mytrek.com. IN   MX     0   turtle.mytrek.com.

Mail addressed to george@mytrek.com would be sent to george@turtle.mytrek.com.

Note

MX records are used not only for incoming mail, but also for outgoing mail. An MX record can specify a mail server to use for relaying mail from a given host out to a larger network.

MX records come into play with certain mail configurations such as masquerading or centralized mail services. MX records are not required. If you have a standalone system or a small network with only a few hosts, you may want mail received directly by different hosts.