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A feedback loop (FBL) is a mechanism that mailbox providers use to notify senders when a recipient marks their email as spam. When a user clicks “Report Spam” or “Mark as Junk” in their email client, the mailbox provider sends a complaint report back to the sender — or in most cases, to the sender’s email service provider. Feedback loops are one of the most important signals in email deliverability. They tell you directly that a recipient does not want your emails, and they give you the opportunity to stop sending to that person before further complaints damage your reputation.

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How Feedback Loops Work

1

Recipient Reports Spam

A recipient clicks “Report Spam,” “Mark as Junk,” or a similar button in their email client (Gmail, Outlook, Yahoo Mail, Apple Mail). This registers a complaint with the mailbox provider.
2

Mailbox Provider Generates a Report

The mailbox provider creates a complaint report in a standardized format — typically ARF (Abuse Reporting Format, defined in RFC 5965). The report includes the original message headers (or a redacted version) and identifies the complaint type.
3

Report Sent to the Sender's ESP

The mailbox provider sends the ARF report to the email address registered in their feedback loop program. For emails sent through Lettr, these reports are sent to Lettr’s feedback loop processing infrastructure.
4

ESP Processes the Complaint

Lettr receives the complaint, identifies the original message and recipient, and records the event. The recipient is automatically added to the suppression list to prevent further sends. The complaint event is fired to your configured webhooks.
5

Sender Takes Action

Your application receives the complaint webhook and can take additional action — updating the recipient’s status in your database, triggering an internal review, or adjusting your sending practices if complaint rates are elevated.

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Which Providers Offer Feedback Loops

Not all mailbox providers operate feedback loop programs, and those that do have varying levels of detail in their reports.
ProviderFBL AvailableFormatNotes
Yahoo / AOLYesARFOne of the original FBL programs. Reports include full message headers.
Outlook.com / HotmailYesARF via JMRP/SNDSMicrosoft’s Junk Mail Reporting Program. Also offers Smart Network Data Services (SNDS) for IP-level reputation data.
GmailPartialNot traditional ARFGmail does not offer a traditional FBL. Instead, it requires senders to implement a List-Unsubscribe header and uses aggregate reputation data via Google Postmaster Tools.
Apple Mail / iCloudLimitedVariesApple processes complaints but does not offer a public FBL enrollment program.
Corporate / EnterpriseVariesVariesMany enterprise email systems (Exchange, Google Workspace) have internal complaint mechanisms that do not generate external FBL reports.
Gmail’s approach is different from traditional feedback loops. Instead of sending individual complaint reports, Gmail expects senders to monitor aggregate data through Google Postmaster Tools and respond to reputation changes at the domain or IP level.

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How Lettr Handles Feedback Loops

Lettr is enrolled in feedback loop programs with major mailbox providers. When a complaint is received, Lettr processes it automatically.

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Automatic Suppression

When a recipient files a spam complaint, Lettr immediately adds their address to your account’s suppression list. This prevents your application from sending further emails to that address, even if your application explicitly requests it. This automatic suppression is critical for protecting your sending reputation. Continuing to send to recipients who have complained is one of the fastest ways to get your domain or IP blocklisted.

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Webhook Events

Lettr fires a email.complained webhook event for every complaint received. Your application can listen for this event to update its own records: 
app.post('/webhooks/lettr', (req, res) => {
  const event = req.body;

  if (event.type === 'email.complained') {
    const { to, complaint_type, timestamp } = event.data;

    // Update your database
    markAsComplained(to, timestamp);

    // Optional: trigger internal review if complaints spike
    checkComplaintRate();
  }

  res.sendStatus(200);
});

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Dashboard Visibility

Complaint events are visible in your Lettr dashboard under email event history. You can filter by complaint events to see which messages generated complaints and identify patterns.

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Complaint Rate and Reputation

Your complaint rate — the percentage of delivered emails that generate spam complaints — is one of the most heavily weighted signals in sender reputation scoring.

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Benchmarks

Complaint RateAssessmentAction
< 0.1%HealthyContinue current practices
0.1% – 0.3%WarningReview recent sends, check content and targeting
0.3% – 0.5%DangerousStop non-essential sends, investigate root cause
> 0.5%CriticalSending reputation is being actively damaged
Google requires bulk senders to maintain a spam complaint rate below 0.3%, and recommends staying below 0.1%. Exceeding this threshold can result in emails being sent to spam or rejected entirely. See Google & Yahoo Sender Requirements.

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Why Complaints Matter More Than Unsubscribes

An unsubscribe is a neutral signal — the recipient used the proper mechanism to stop receiving emails. A complaint is a negative signal — the recipient told their mailbox provider that your email is spam. From a reputation perspective, complaints are far more damaging than unsubscribes. This is why making it easy to unsubscribe is a deliverability strategy, not a subscriber retention risk. Every recipient who unsubscribes instead of complaining is protecting your reputation.

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Reducing Complaint Rates

If the unsubscribe link is buried at the bottom of your email in small gray text, recipients will use the more visible “Report Spam” button instead. Place your unsubscribe link prominently. Implement one-click unsubscribe via the List-Unsubscribe header — Lettr supports this automatically.
The most common cause of high complaint rates is sending to people who did not explicitly agree to receive your emails. Never add recipients without their consent. Use double opt-in to confirm that the address is valid and the owner wants your messages.
Tell subscribers what kind of emails they will receive and how often. If someone signs up expecting a single welcome email and receives weekly promotions, they will complain. Align your sending behavior with what you promised.
Sending irrelevant content increases complaint rates. Segment your audience and send content that matches their interests and engagement level. A user who signed up for product updates does not necessarily want marketing promotions.
Process unsubscribe requests in real time. If a recipient unsubscribes and receives another email the next day because of processing delays, they will often file a spam complaint. Lettr processes List-Unsubscribe requests immediately.

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The ARF Format

For technical readers, the Abuse Reporting Format (ARF) defined in RFC 5965 is the standard format for feedback loop reports. An ARF message is a MIME multipart message containing three parts:
  1. Human-readable description — A plain text summary of the complaint
  2. Machine-readable report — Structured fields including Feedback-Type, User-Agent, Version, and Arrival-Date
  3. Original message — The original email (or its headers) that was reported
Common Feedback-Type values:
TypeMeaning
abuseRecipient reported the message as spam
fraudMessage is suspected phishing or fraud
not-spamRecipient indicated the message is not spam (rare)
otherOther complaint type
You do not need to parse ARF reports yourself. Lettr processes all incoming feedback loop reports and translates them into structured webhook events and dashboard data.

What Is Email Deliverability?

How feedback loops fit into the broader deliverability picture.

Sending Reputation

How complaint rates affect your sender reputation and what to do about it.

Unsubscribe Best Practices

Implement unsubscribe mechanisms that reduce complaints and protect reputation.

Suppression Lists

How Lettr manages suppressed addresses from bounces and complaints.