I came across the following diagram at this site, and it nicely summarizes the issue of backscatter spam:
Getting a single piece of backscatter spam is one thing, getting dozens, hundreds or even thousands of them is a major problem. While spammers may be nefarious in attempting to spam indirectly, what's more annoying is that legitimate hosts are sending you piles of messages that are cluttering up your inbox.
So why is backscatter so difficult to defend against? Here are some reasons:
That summarizes the problem of backscatter. Relying on regular inbound mail filtering to detect and filter backscatter introduces problems because NDR messages are different than regular mail. They are notifications. In order catch them you need to do something different than the way you normally catch spam.
PingBack from http://blog.a-foton.ru/2008/07/the-problem-of-backscatter-part-8-why-is-it-so-hard-to-stop/
I read an article about a new technology called ReceiverNet from Abaca. ReceiverNet technology characterizes each protected user based on the percentage of spam they receive and then uses those reputations to rate the incoming message flow. I changed my spam filtering system to Abaca’s Email Protection Gateway and it blocked Replica watches spam mails, Subpoena Phishing mails and many more. I found that Abaca’s ReceiverNet service has 99% efficiency in blocking spam mails and they guarantee their results. For more information, log on to http://abaca.com/.