Feedback loop
An ISP program that reports back which of your recipients marked your email as spam.
A feedback loop (FBL) is an ISP-run program that reports back to senders when their recipients mark messages as spam. Gmail's FBL is opt-in for senders who meet volume thresholds; Microsoft offers similar programs (JMR, SNDS); Yahoo and others have equivalent mechanisms. Without FBLs, senders have no direct visibility into spam complaints — the only signal is degrading deliverability.
Most ESPs (Mailchimp, Klaviyo, SendGrid, Postmark) sign up for FBLs on behalf of their customers and automatically suppress complainers from future sends. Senders running their own infrastructure must sign up directly and process the FBL emails programmatically.
The most important FBL action is permanent suppression of complainers. Continuing to send to recipients who have already complained is the single fastest way to accumulate complaints at scale, which compounds the reputation damage. Process FBLs in near-real-time and respect them as hard suppressions.