{
 "id": 69778,
 "slug": "backscatter-spam-bounce-messages-abused-by-spammers",
 "url": "https://agents.willem.com/en/snippets/backscatter-spam-bounce-messages-abused-by-spammers/",
 "language": "en",
 "title": "Backscatter spam: bounce messages abused by spammers",
 "subtitle": "Why a well-behaved mail server can still spam strangers.",
 "summary": "Backscatter spam is incorrectly automated bounce messages sent by a mail server, usually as a side effect of incoming spam: spammers forge the sender address, so the bounce delivers their message to an innocent third party.",
 "topics": [
  "cloud",
  "cyber-security",
  "howto",
  "linux",
  "networking",
  "server"
 ],
 "date": "2019-09-10",
 "published": "2019-09-10T00:00:00+02:00",
 "created": "2026-08-30T07:13:28.285000+02:00",
 "updated": "2026-08-30T07:16:52.917000+02:00",
 "full_article_url": "https://willem.com/en/2019-09-10_fighting-backscatter-spam-at-server-level/",
 "author": {
  "name": "Willem L. Middelkoop",
  "url": "https://willem.com"
 },
 "note": "A short distillation of a longer willem.com post. Not currently published on willem.com itself.",
 "answers_question": "What is backscatter spam?",
 "text": "Spammers send messages they expect to be bounced, with somebody else's address forged as the sender. When the message bounces, the server dutifully returns it to the falsified sender and effectively delivers spam for them.\nThe simple scenario is solved by never bouncing: respond with an SMTP error code during the connection instead. The hard scenario is forwarding. A user on your server forwards mail to another server whose stricter spam filter blocks the message, and your server then informs the forged original sender that forwarding failed. You do not control that second server, and the delay between accepting and forwarding makes waiting for its verdict impossible.\nSend enough of this and your server lands on the backscatterer.org blacklist, after which other servers treat your legitimate messages as suspicious.",
 "markdown_url": "https://agents.willem.com/en/snippets/backscatter-spam-bounce-messages-abused-by-spammers/snippet.md"
}