Glossary · Deliverability
A feedback loop is a mailbox provider's service that notifies a sender when recipients mark its mail as spam.
Senders register their IPs or domains with a provider's programme, and each spam complaint comes back as a report, usually in the Abuse Reporting Format (RFC 5965). Microsoft and Yahoo run per-message programmes. Gmail instead shows aggregate complaint rates in Postmaster Tools.
The right response to a complaint is to stop mailing that person immediately. Complaint rate is one of the strongest inputs to sender reputation; Gmail and Yahoo ask bulk senders to stay well below 0.3%.
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