A feedback loop is a service run by a mailbox provider that tells senders when its users mark their email as spam. When someone clicks “Report spam”, the provider sends a report, often in the standard ARF format, to an address the sender registered. The sender can then unsubscribe that person straight away so they never receive another marketing email.
Different providers run feedback loops in different ways. Yahoo offers a complaint feedback loop for domains that sign with DKIM, and Microsoft runs a programme for its consumer mail services. Gmail does not report individual complaints. Instead, senders can add a Feedback-ID header to their emails, and Gmail shows aggregate spam rates for those identifiers in Postmaster Tools, which helps you see which campaign or stream is causing complaints without naming the people who complained.
Email platforms register for the feedback loops that are available and process the reports for you, so most businesses never handle them directly. If you send through your own sending service, it is worth checking that complaints are being passed back to your platform, usually through a bounce and complaint webhook, so that suppression actually happens.
The value of feedback-loop data goes beyond suppression. A spike in complaints after one campaign points to a specific problem: a segment that didn’t expect the email, a misleading subject, or a sudden jump in frequency. Looking at complaints by campaign and by source of sign-up often reveals a bad list source faster than any other metric.
A Ludhiana sportswear brand sends through its own Amazon SES account. Complaint notifications from SES are forwarded to its email platform by webhook, and each complainant is automatically suppressed. Reviewing the complaints, it notices most came from contacts imported from a trade show, and stops mailing that segment.
MailNesto receives bounce and complaint notifications from sending services such as Amazon SES, SendGrid, Mailgun and Brevo through webhooks, stops emailing people who complain, and adds a Feedback-ID header to campaign emails.