module to query multiple Realtime Blackhole Lists in parallel

 The Net::RBLClient 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
 RBLs, you should carefully study the behavior of those RBLs before and
 during such blocking.