Challenge Response and Backscatter One anti spam proposal has been the challenge response type of system which sends challenge messages to unknown senders of incoming mail and awaits a response before delivering a message Such systems are generally declared as failures and backscatter is the reason Concerning a legitimate message the challenge would typically be sent to the sender However for spam this is often a forged address so such a challenge raises the noise floor for its innocent recipient Although such a recipient will fail to reply thus causing the originating spam to be discarded one is effectively placing the decision into the hands of an unknown third party who is annoyed by the unsolicited challenge message Furthermore for a challenge to be useful it must somehow refer to the original message sent and even include a partial copy of such When a C R system does so effectively it becomes a spam relay A spammer knowing that a certain C R system includes a copy will design his message so that the spam payload is among the copied portion and forge his desired recipient s address as if he were the sender thus allowing the C R system to be the actual delivering server of the spam as a challenge message payload to his desired target The definition of backscatter as used by some individuals very well includes such challenges from C R systems especially those which can be tricked by spammers to deliver their message payloads Such challenges are not technically bounce messages as they are not NDRs but otherwise satisfy all the other common requirements to be considered backscatter if not spam in its own right 71 106 213 194