Most Java developers have never needed to interact with the DNS directly; the most well-known job of DNS, that of resolving hostnames to IP addresses, is performed automatically every time a network operation involving a hostname is performed. Similarly, finding a server that is willing to accept email for a particular email address is performed transparently using DNS. How would a Java developer get access to such arcane, low-level information to do something as simple as validate that a particular email address has a server somewhere in the world willing to accept email for it? It all comes down to hierarchy.