A feedback loop tells senders who marked their mail as spam so those people can be removed.
Key points
- RFC 6449 describes how complaint feedback loops work and what senders and providers are expected to do with the reports [1].
- Reports are usually sent in the Abuse Reporting Format (ARF) defined in RFC 5965, which wraps the original message with machine-readable complaint details [2].
- Yahoo's Complaint Feedback Loop is domain based and only covers mail signed with DKIM (DomainKeys Identified Mail) [3].
- Gmail's FBL does not reveal individual complainers; it uses a Feedback-ID header and reports campaigns with unusual spam rates in Google Postmaster Tools [4].
- Every complaint received through an FBL should be added to a Suppression List right away to protect your Spam Complaint Rate [1][3].
How a feedback loop works
When a recipient clicks the spam button, the mailbox provider records the complaint and uses it in its own filtering. If the sender has enrolled in that provider's feedback loop, the provider also sends a report to an address the sender registered. RFC 6449 sets out the operating model: senders apply to each provider, prove they control the domain or IP ranges, and commit to acting on reports promptly [1]. The report itself typically follows RFC 5965, the Abuse Reporting Format, a MIME message that includes a human-readable part, a machine-readable part with fields such as Feedback-Type, and a copy of the original message or its headers [2]. Providers often redact the recipient address, so senders embed their own identifiers in Email Headers to match a report back to a contact.
Provider differences
Feedback loops are not uniform. Yahoo's program requires senders to create a sender profile, verify a domain and enroll it, and it only reports on DKIM (DomainKeys Identified Mail)-signed mail, since the signing domain identifies who is responsible [3]. Gmail takes a different approach: it does not send individual reports. Large senders add a Feedback-ID header with up to four identifiers, such as a campaign or customer ID, and Gmail flags identifiers with unusually high spam rates in its Postmaster Tools dashboard [4]. That traffic must be DKIM-signed by a domain registered in Postmaster Tools, covered by SPF (Sender Policy Framework) and sent from IPs with valid Reverse DNS (PTR Record) [4]. Wikipedia keeps a broader overview of which providers run FBLs [5].
What senders should do with complaints
The core rule is simple: stop mailing anyone who complains. Add them to a global Suppression List so no future campaign, sequence or import reaches them again. Beyond individual removals, patterns in FBL data are useful diagnostics. A cluster of complaints on one Email Sequence step, one segment or one sending domain points to targeting or copy problems that will otherwise surface as falling Inbox Placement. Since FBL coverage is partial and Gmail only shares aggregates, complaint data should be read together with Bounce Rate, replies and Postmaster Tools trends rather than on its own [1][4]. For low-volume Cold Email senders, FBL reports are rare, which makes each one worth investigating.
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