What is backscatter spam?AnswerBackscatter 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.SourcesBackscatter spam: bounce messages abused by spammers (snippet, the answer above is its summary)https://willem.com/en/2019-09-10_fighting-backscatter-spam-at-server-level/ (full article)