Had to look up what a backscatter zombie was; apparently it's what happens when you accept bogus mail (spam) into your mail queue without checking if it is actually deliverable -- thus end up bouncing it once the queue is processed for delivery -- and typically then sending it back to a forged address -- ie: you become a spam gateway (spammers send you email with sender-addresses they want you to try to send spam to...):
http://serverfault.com/questions/111938/how-might-i-stop-bac...