One of the methods to block off junk mails sent by spammers with e-mail servers that are connected to the Internet by means of dial-up connection, cable and ADSL connections is the DNS lookup. The IP addresses of those types of connections are not registered, usually, to a DNS as a legitimate host. This means that they do not have a
registered host name and their very own static IP.
In a DNS lookup, the Internet domain name is used to look for the IP address. Most spammers usually use hosts that are purposely misconfigured to hide the source of incoming spam. When a query on the DNS lookup doesn't come back with
host name that matches with the IP address, it's a tell-tale sign that the message is junk or spam.
Using DNS lookup isn't exactly the best solution. There are some
e-mail servers that are legitimate but have poorly configured or may not have intentionally registered their name with DNS. So when a reverse query is done, the result will reveal that the host name is not matched. This method use up a valuable amount of network resources and network delays could cause problems.
