Feedback LoopFeedback Loop (FBL)
A feedback loop (FBL), also called a complaint feedback loop, is a service from a mailbox provider that tells a sender when its recipients mark a message as spam. When someone hits “Report Spam,” a registered sender receives a copy of the complaint so it can suppress that recipient at once. It is how you find out about complaints you would otherwise never see, and acting on it is essential to protecting your reputation.
- Reports back which recipients marked your mail as spam
- You must register with each provider to receive its reports
- Most reports arrive in the standard ARF format (RFC 5965)
- Gmail gives only aggregate complaint rates, not individual complainers
How a feedback loop works
When a recipient clicks “Report Spam,” “Junk,” or the equivalent button, their mailbox provider records a spam complaint against the sender. Without a feedback loop, that complaint is invisible to you; it quietly damages your reputation while you keep mailing the person who reported you. A feedback loop closes that gap: a sender who has registered with the provider receives a copy of each complaint, typically by email.
The point of the loop is action. The moment a complaint comes in, you should add that recipient to your suppression list and stop mailing them, because continuing to send to someone who has already reported you is the fastest way to rack up more complaints and sink your reputation. A working FBL turns invisible damage into a list you can act on.
The ARF format
Most feedback loops deliver complaints in the Abuse Reporting Format (ARF), standardised in RFC 5965. An ARF report is a structured email with three parts: a human-readable summary, a machine-readable section with the complaint details, and a copy of the original message (or at least its headers) so you can identify the recipient and campaign.
Content-Type: message/feedback-report
Feedback-Type: abuse
User-Agent: SomeProvider-FBL/1.0
Version: 1
Original-Mail-From: bounce@example.com
Original-Rcpt-To: reporter@mailbox.example
Arrival-Date: Wed, 10 Jun 2026 09:14:00 +0000How the providers differ
Each major mailbox provider runs its feedback program a little differently, and the differences matter:
- Yahoo (CFL). Yahoo’s Complaint Feedback Loop sends individual complaints in ARF. You apply for it, and because Yahoo keys the loop on your DKIM signature, your mail must be DKIM-signed to enrol.
- Microsoft (JMRP). The Junk Mail Reporting Program forwards a copy of each message a user marks as junk. You configure it through Microsoft’s SNDS portal, registering the sending IPs you want covered.
- Gmail. Google does not provide a traditional per-complaint FBL for most senders. Instead, Postmaster Tools reports an aggregate spam rate, with no individual complainer data, to protect user privacy. A separate Gmail FBL for ESPs reports per-campaign rates, again in aggregate.
The practical upshot: with Yahoo and Microsoft you can identify and suppress the exact people who complained, while with Gmail you can only watch the overall rate and react to trends. Google asks bulk senders to keep that rate under 0.3% in Postmaster Tools.
How a complaint travels through a feedback loop
Feedback loops by provider
| Yahoo (CFL) | Microsoft (JMRP) | Gmail | |
|---|---|---|---|
| Report type | Individual | Individual | Aggregate rate |
| Format | ARF | ARF | Postmaster Tools |
| Identifies complainers | Yes | Yes | No |
| Keyed on | DKIM domain | Sending IP (via SNDS) | Domain / IP |
| Sign-up | CFL application | SNDS then JMRP | Postmaster Tools |