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).

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