Identify hosts that you don't want on your site. Try it with:
$ curl 'http://v1.nastyhosts.com/52.30.0.1' { "status":200, "asn":{ "asn":"16509", "name":"Amazon.com, Inc." }, "country":{ "code":"US", "country":"United States" }, "hostnames":[ "ec2-52-30-0-1.eu-west-1.compute.amazonaws.com" ], "suggestion":"deny" }
NastyHosts.com was designed for web applications and tries to detect all IP addresses from VPS clouds, VPNs and known spammy networks. It CANNOT be used to test mail servers.