Every day, your users click “This is spam”. Too often, that click goes nowhere: the network that sent the spam never hears about it, and the same mail keeps coming. Complaint feedback loops (FBLs) fix that. They send each complaint back to the network that sent the message, so it can act. 

This guide explains how FBLs work, why they matter for both receivers and senders, and how to set one up in minutes.

What is a complaint feedback loop?

A feedback loop is a way for a mailbox provider to tell a sender that a recipient marked their message as spam. Each complaint is packaged as a report, usually containing the original message headers, and sent to the sender’s abuse or postmaster address.

FBLs have been used since the mid-2000s. For most of that time, they relied on a central service run by Return Path that collected complaints from receivers and forwarded them to registered senders.

How a feedback loop works

1. A recipient complains. They click “This is spam” or “Report junk” in their email app.
2. The receiver registers the complaint and creates a report with the message headers and metadata. Personal details can be removed for privacy.
3. The report goes to the sender, either directly or through a reporting service, using the address the sender has published.
4. The sender acts. They find the account, campaign, or system responsible and fix the problem.
5. The loop closes. Less spam is sent, fewer complaints arrive, and legitimate mail is less likely to be blocked.

What is in a complaint report?

Most complaint reports use the Abuse Reporting Format (ARF), defined in RFC 5965. Because the format is standard, reports can be processed automatically. A typical report includes:

• A short human-readable summary of the complaint.
• Machine-readable details, such as the feedback type (“abuse”), the reporting system and the source IP address.
• The original message, or at least its headers, so the sender can trace which account or campaign sent it.

Why feedback loops matter now

In 2023, Validity, which acquired Return Path, announced it would start charging senders to receive reports from its Universal Feedback Loop. That put basic email hygiene behind a paywall.

The email community responded with an open standard. RFC 9477, published in 2023, lets senders say where complaint reports should go, using a header tied to their DKIM signature. A related Internet Draft does the same with DNS records. Any receiver can now send reports to any sender, without a middleman.

The catch is that receivers and senders both have to implement it, and that takes time. That is where Abusix comes in.

For email receivers: why encourage spam complaints?

If you run mailboxes for users, as an ISP, hosting provider, telecom or business, sharing complaints back to senders helps you directly:

• Less spam reaches your users. Senders hear about problems on their networks quickly and stop them sooner.
• Fewer support tickets. When users see that reporting spam works, they report it instead of contacting your support desk.
• Less blocking of legitimate mail. Senders who fix problems keep their reputation, so you need to block less.

How to send your complaints through Abusix

Abusix runs a free FBL reporting service. You forward your users’ complaints to us. We package them correctly, check that the sender is trustworthy enough to receive a report using our reputation data, and send the report to any sender that has implemented RFC 9477 or the Internet Draft.

Setup takes a few minutes:

1. Create an account at app.abusix.com and log in.
2. Under “Platform Functions”, select “Data Channels”, then “Create a Data Channel”.
3. Choose “Report Abusive Behaviour”, then “Send data using SMTP forward”.
4. Name the channel (for example “Feedback Loop”) and select “Mail from a spam inbox”.
5. Accept the terms of service. You will receive an SMTP address for your reports.
6. Point your mail platform’s “This is spam” button at that address.
7. Email [email protected] so we can enable the channel for feedback loop reporting.

Already sending complaints to Validity? You can send them to us at the same time. At worst, a sender receives two reports.

For email senders: why sign up for feedback loops?

If you send mail, as a hosting provider, ESP, SaaS platform, or business, complaint reports are an early warning system:

• Spot compromised accounts fast. A sudden rise in complaints often means an account, server or device has been hijacked.
• Find poor sending practices. Complaints point to customers or campaigns with bad address collection or unwanted mail.
• Protect your reputation. Acting on complaints keeps your IP addresses and domains off blocklists.

How to receive complaint reports

To receive reports from receivers that support the new standards, including through Abusix, do one or both of the following:

• RFC 9477: add a DKIM-aligned CFBL-Address header to every message you send, naming the address where reports should go.
• Internet Draft: publish the additional DNS TXT records the draft describes.

Doing both gives the widest compatibility and causes no problems.

What to do with the reports

• Track complaints per account, customer or campaign, and compare them with how much mail each one sends.
• Investigate any sudden spike straight away: reset passwords, scan for malware and pause sending if needed.
• Remove complainers from mailing lists so they don’t receive further mail.
• Act on repeat offenders under your acceptable use policy.

Because reports use ARF, you can process them automatically. Abusix Ops (formerly known as AbuseHQ) can process them for you alongside your other abuse reports.

A note on regional differences

Feedback loops are widely offered by large US mailbox providers. In Europe, availability varies more by country and provider. If you send to European recipients, check which providers offer feedback loops, and encourage those that don’t to adopt the new standards.

Help close the loop

Feedback loops only work when both sides take part. Receivers: forward your complaints. Senders: publish where you want reports to go. And ask the mailbox providers you work with to support the open standards, or to send their complaints through us.

Set up your free feedback loop reporting, or learn how to check if your server is sending spam.