Feedback Loop (FBL)
A service from a mailbox provider that forwards spam complaints back to the sender so they can suppress the complaining recipient.
A feedback loop (FBL) is a programmatic channel through which mailbox providers (Yahoo, Microsoft, Comcast, La Poste, and many others) forward `this is spam` complaints back to the originating sender. The sender enrolls per IP or per domain, receives the complaint as an ARF-formatted report (RFC 5965), and is expected to suppress that recipient from future sends. FBLs are essential for keeping complaint rates below mailbox-provider thresholds (typically 0.1 to 0.3 percent), and ignoring them is one of the fastest ways to land on a deny list.