WTF Series
WTF is Feedback Loop?
The system that tells you when someone marks your email as spam. Most senders don't even know it exists.
The real explanation
A feedback loop (FBL) is a service offered by mailbox providers (like Yahoo, Outlook, AOL) that sends you a notification when a recipient hits the "Report Spam" or "Mark as Junk" button on your email. Gmail works differently and uses its Postmaster Tools instead of traditional FBLs, but the concept is the same: you get signal that real people are rejecting your email.
Here's the part that frustrates people: your ESP should be handling FBL signups and automatically suppressing complainers. Most do. But not all of them tell you about it. You might have a complaint rate problem and never know because your ESP quietly removes complainers without surfacing the data. Ask your ESP if they process FBLs and where you can see complaint rates.
The industry benchmark is to stay below 0.1% complaint rate. Gmail's threshold is even stricter at 0.3% as a hard ceiling and 0.1% as the "you should worry" line. One bad campaign to a cold segment can push you over in a single send.
Show me an example
You send a promotional blast to your entire list, including people who haven't opened in 6 months. 200 of them hit "spam" instead of unsubscribing. Your complaint rate spikes to 0.4%. Gmail starts routing your emails to spam for everyone, even the engaged subscribers. Your ESP processed the FBL reports but you didn't notice the spike until a week later when your open rates cratered.
Who handles this?
Reef covers reputation & blocklists.
Go deeper
Read more in the Email Almanac: Feedback Loop in the Almanac
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