FEEDBACKLOOP
Feedback loop
Feedback loops are one of the ways for reporting spam. Whether and how to provide an FBL is a choice of the MP. End users should report abuse at their mailbox provider's reporting hub, so as to also help filtering. As an alternative, competent users may send abuse complaints directly, acting as mailbox providers themselves.The above text is a snippet from Wikipedia: Feedback loop (email)
and as such is available under the Creative Commons Attribution/Share-Alike License.