This module is used to discover what RBL's are listing a particular
IP address. It parallelizes requests for fast response.

An RBL, or Realtime Blackhole List, is a list of IP addresses meeting
some criteria such as involvement in Unsolicited Bulk Email. Each RBL
has its own criteria for addition and removal of addresses. If you
want to block email or other traffic to/from your network based on one
or more RBL's, you should carefully study the behavior of those RBL's
before and during such blocking.

Author:	Asher Blum <asher@wildspark.com>
WWW:	http://search.cpan.org/dist/Net-RBLClient/