Mail exchange records and other types of resource records are the basic information elements of the DNS and are differentiated through type identification like MX, NS, A, etc., together with a DNS class. These records have a specific validity period before the information within them must be refreshed by an authoritative name server.

In its simplest form, a domain may have a single mail server, so when a mail transfer agent (MTA) queries the MX records for www.something.com for a mail server and the DNS replies with only one mail server, mail.something.com, even with a large preference number like 60, the MTA will select this single mail server for email delivery. In this case, it would not matter what the preference number is because there is only a single mail server.