Glossary
Feedback loop
Also called FBL, complaint feedback loop.
Last reviewed 18 September 2026
What is an email feedback loop?
A feedback loop is an arrangement in which a mailbox provider reports back to a sender the messages its users marked as spam, so that those addresses can be suppressed immediately rather than mailed again.
Not every provider offers one and the terms differ: some report the complaint with the address and some strip it, and Gmail runs no per-message loop of the ordinary kind at all, offering aggregate rates in its postmaster dashboard instead. Most sending platforms enrol in the loops that exist and act on them without being asked.
What a loop is for is the one thing worse than a complaint, which is a second message to somebody who already complained. An address that arrives through a feedback loop belongs on the suppression list the same day, and it never comes back off it.
See also
- Spam complaint rate
A spam complaint rate is the count of people who pressed the spam or junk button divided by the mail delivered to that provider over a window it chooses, and Google and Yahoo both name 0.3 per cent as the level to stay under and 0.1 per cent as the level to aim at.
- Suppression list
A suppression list is the permanent record of addresses that must never be mailed again, holding unsubscribes, complaints and hard bounces, and it is checked before every send rather than after one.
- Sender reputation
Sender reputation is what a mailbox provider has learned about the domain and the infrastructure your mail comes from, assembled from complaints, bounces, spam trap hits and how the people who receive your mail behave, and it is what decides the folder.
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