add handling of UNREGISTERED (drop mail on cropswap-members group)

filters/cropswap-members@shandan.one

@@ -20,6 +20,15 @@ if ($DISPATCH)
    {
       EXITCODE=$UNREGISTERED
       cc "|$DEBUGMAIL unregistered"
+      # this is not temporary error
+      # and we send no response to
+      # unregistered email as the
+      # email address could be:
+      #   - Mail Delivery Subsystem
+      #   - DoS attacker
+      # so mark as successful delivery
+      # effectively dropping the mail
+      EXITCODE=0
       exit
    }