OpenSMTPD filter integration for SenderScore reputation

 This filter performs a SenderScore lookup and allows OpenSMTPD to either block
 or slow down a session based on the reputation of the source IP address.
 .
 The filter currently supports:
  * blocking hosts with reputation below a certain value
  * adding an X-SenderScore header with the score of the source IP address
  * adding an X-Spam header to hosts with reputation below a certain value
  * applying a time penalty proportional to the IP reputation
  * whitelisting IP addresses or subnets