What is a feedback loop (FBL)?

Feedback loop (FBL) is a mechanism where major ISPs notify senders when their recipients click "Mark as spam" on a message. The ISP forwards the original message (with headers) to a designated address at the sender, so the sender can identify which addresses complained and suppress them automatically.

Major ISP feedback loops:

• Yahoo, AOL, and Verizon Media share a combined FBL.
• Microsoft (Outlook.com, Hotmail, Live) runs JMRP (Junk Mail Reporting Program).
• Comcast, Cox, and USA.net run their own FBLs.
• Gmail has no traditional FBL. It uses postmaster tools instead for aggregate complaint data.

Most reputable ESPs are automatically subscribed to these FBLs and process the reports for their customers. Complaining addresses are added to the suppression list automatically. If you self-host SMTP, you need to subscribe to each FBL individually.

Why this matters: complaint rates above 0.3% are catastrophic for sender reputation. FBLs give you direct visibility into the complaint pattern so you can suppress the complainers and investigate what triggered them. Without FBL processing, complaints accumulate silently and your reputation degrades without any obvious cause.