A feedback loop (FBL) is a service offered by mailbox providers that notifies a sender whenever a recipient marks one of their emails as spam.
In depth
When you enrol your sending domain or IP in a provider’s FBL, each spam complaint is reported back to you, usually with the complaining address redacted or hashed. The correct response is to suppress that recipient immediately — continuing to mail someone who has complained is one of the fastest ways to damage sender reputation. Most large providers offer FBLs; Google uses a different, aggregate scheme rather than per-message complaints.
Example
If Microsoft’s FBL reports that a subscriber marked your newsletter as spam, you add them to your suppression list and never email them again.
Related terms
See our full email marketing glossary or the complete email marketing guide.