Also called: FBL
A feedback loop (FBL) is a mailbox provider's system for reporting back to a sender when a recipient marks their mail as spam, so the sender can see complaint volume and remove those addresses instead of finding out only through a damaged reputation.
A feedback loop, often abbreviated FBL, is how a mailbox provider tells a sender that a recipient just hit the spam or report button on one of their messages. Yahoo runs its own Complaint Feedback Loop under that name. Gmail runs the same function through its own user-report learning system, paired with bounce and inactive-recipient tracking. Neither requires the sender to guess at complaint volume from the outside. Both hand it back directly, if the sender is set up to receive it.
Complaint feedback is one of the few signals mailbox providers score no matter how the message got written, by a person or drafted with AI help. A well-targeted email that annoys enough recipients gets the same reputation hit as a sloppy one that annoys the same number of people, because the filter is reading recipient reaction, not judging the copy.
That makes the feedback loop a direct line to the number that actually damages sending reputation: complaint rate. Bounce rate says a list has bad addresses on it. Complaint rate says real people are seeing the mail and choosing to report it, which mailbox providers treat as a stronger, more deliberate negative signal than a bounce.
Route feedback-loop reports into the same suppression process as unsubscribes and hard bounces. Anyone who complains gets removed from every future send, not just the campaign that triggered the complaint, since re-mailing a person who already reported the last message all but guarantees a second complaint.
Teams watch open rate and reply rate closely and barely look at complaint rate, because it doesn't show up as a headline metric in most sequencer dashboards. But complaint rate is the number Google and Yahoo actually cap for bulk senders, and a program optimizing for replies while ignoring complaints can be quietly building the exact reputation problem that later tanks its inbox placement.
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