Worms and other malicious software sending email from an address without the address owner's knowledge can also result in unexpected delivery failure messages.
What Causes Backscatter?
If you mistype an email address and delivery is not possible, you get back a delivery failure report from a mail server letting you know that and why your email could not be put into the recipient's mailbox.
Sending junk email, spammers can masquerade as you in their email messages. When delivery fails, a delivery failure report is returned — not to the spammer, though, but to you as your email address appeared as the sender.
Is Backscatter Spam?
Often, delivery failure reports reproduce the original message. At the same time, these usually helpful messages are, for the most part, not filtered as spam by junk mail filters. This makes backscatter a viable way to distribute spam.
Spam filters looking at the returned original message in delivery failure reports independently from the status message can successfully block backscatter spam.