What is Backscatter?

Chris writes:   I have my own email server that I set up for a few friends and myself.  I've been trying to cut the amount of Spam my server gets, but now my emails are being rejected with a backscatter.org message.  What is backscatter and how can I stop it?
Hi Chris,

Postfix: Fighting Spam using a Real-Time Block List (RBL)

     For those that administer postfix email servers that see high volumes of email, dealing with Spam can be quite a challenge. Initially I was relying on the server to do its own Spam checking with tools such as amavisd-new, spamassassin, etc. Which was working really well for the most part, but when the Spam volume gets high, performing all that processing, in the hundreds of thousands of emails, was putting unnecessary load on the server.