What is this tool?
This test will see if a reverse DNS entry exists for an IP address, and will also show you how the entry is found (the route of DNS servers that is taken), and who to contact to get a reverse DSN entry if none is found. The RFCs say that you should have a reverse DNS entry for every host on the Internet. If a mail server is missing a reverse DNS entry, other mail servers may reject mail from it. It supports IPv6 reverse DNS lookups, too.
How do the results help me?