This great blog post by the Forefront team about Backscatter protection and how to deal with it using Forefront Protection 2010 for Exchange Server.
In essence, backscatter is a DSN (Delivery Status Notification) delivered to a recipient who never sent the original mail in the first place. While the DSN itself is perfectly legitimate and valid notification initiated by a reputable MTA, it has been sent to a user who never initiated the original mail transaction.
Read the blog post here.