Based on recent discussions, I would suggest an update to the requirements:
1.    must minimize unwanted messages to some acceptable level
2.    must not affect delivery(latency, integrity, cost, reliability) of
      wanted messages to a point that would effect the normal use of email
  2a) must not require that any message be accepted or rejected
  Much of the discussion around anti-spam systems indicates that most
people think that anti-spam systems are "either-or" type of systems.
i.e. As a result of applying the system to the message, it is either
accepted or rejected.  I would like to discourage such thinking.
  Anti-spam systems should be designed to give the local administrator
more information with which to make accept/reject decisions.  The
systems themselves hould not require that the adminstrator do anything
with the message, as the action taken after deciding a message is spam
is entirely up to local site policy.
  Alan DeKok.
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