DNS provides two principal services, it performs name to address mapping (and also maps addresses to host names), as discussed in "Name-to-Address Resolution". It also helps mail delivery agents, such as sendmail and POP, deliver mail along the Internet.
To deliver mail across the Internet, DNS uses mail exchange records (MX records). Most organizations don't allow direct delivery of mail that comes across the Internet for hosts within the organization. Instead, they use a central mail host (or a set of mail hosts) to intercept incoming mail messages and route them to their recipients.
The mail exchange record identifies the mail host that services each machine in a domain. Therefore, a mail exchange record lists the DNS domain names of remote organizations and either the IP address or the host name of its corresponding mail host.