A feedback loop (FBL), sometimes called a complaint feedback loop, is an inter-organizational form of feedback by which a Mailbox Provider (MP) forwards the complaints originating from their users to the sender's organizations. MPs can receive users' complaints by placing report spam buttons on their webmail pages, or in their email client, or via help desks. The message sender's organization, often an email service provider, has to come to an agreement with each MP from which they want to collect users' complaints.[1]

Feedback loops are one of the ways for reporting spam. Whether and how to provide an FBL is a choice of the MP. End users should report abuse at their mailbox provider's reporting hub, so as to also help filtering.[2] As an alternative, competent users may send abuse complaints directly, acting as mailbox providers themselves.