The domain name system (DNS) is the phone book of the Internet: it tells computers where to send and retrieve information. Unfortunately, it also accepts any address given to it, no questions asked.
Email servers use DNS to route their messages, which means they’re vulnerable to security issues in the DNS infrastructure. Researchers found that emails supposed to be sent through Yahoo!, Hotmail, and Gmail servers, could be routed instead through malicious mail servers. Attackers were exploiting a vulnerability in the Domain Name System (DNS)—it didn't check for credentials before accepting an answer.
The solution is a protocol called DNSSEC; it adds a layer of trust on top of DNS by providing authentication. When a DNS query is looking for metric.international, the name servers help to verify the records returned for Metric International, and helps verify the records returned for the website. The DNS name servers help verify the website, and information published is vetted by a thorough security procedure.