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.