Backscatter
Bounces, auto-replies and other notices sent to an innocent address that spammers forged as the sender, flooding it with reports about mail it never sent.
What it means
Spam often carries a real address as its forged sender. When such a message is accepted by a server that later cannot deliver it, or reaches an automatic responder, the bounce or reply goes to the forged address. Microsoft defines backscatter as non-delivery reports received for messages you did not send, and Postfix's guide describes innocent sites flooded with undeliverable-mail notifications.
Servers that send backscatter get listed: Backscatterer.org lists IPs that send misdirected bounces, autoresponders or sender callouts. The cure is on their side: reject invalid recipients during the SMTP session instead of accepting and bouncing, and do not bounce mail already judged to be spam, as RFC 5321 also advises for hostile content. Domain owners reduce the forgery itself with SPF, DKIM and an enforced DMARC policy.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Microsoft Learn: Backscatter in cloud organizationslearn.microsoft.com
- Postfix Backscatter Howtopostfix.org
- Backscatterer.org: listing policybackscatterer.org
- RFC 5321, section 6.2: Unwanted, Unsolicited, and "Attack" Messagesrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes