Backscatter (also known as outscatter, misdirected bounces, blowback or collateral spam) is incorrect automated bounce messages sent by mail servers, typically as a side effect of incoming spam.
Recipients of such messages see them as a form of unsolicited bulk email or spam since they were not solicited by the recipients, are substantially similar to each other and are delivered in bulk quantities. Systems that generate email backscatter can end up being listed on various DNSBLs and be in violation of internet service providers' Terms of Service.
Backscatter occurs because worms and spam messages often forge their sender address, and mailservers configured by naive administrators send a bounce message to this address.
Measures to reduce the problem include avoiding the need for bounce message by doing most rejections at the initial SMTP connection stage. This is how WDDX.NET mail servers behave, using Recipient Validation to determine if an account is real or not.