What is a feedback loop for email?

A feedback loop is a service some mailbox providers offer that tells a sender when one of their recipients marks an email as spam, so you can act on the complaint instead of being blind to it:

• What it gives you - a notification each time someone hits the spam button, letting you remove that person before they complain again.
• Who runs them - providers like Microsoft (Outlook, Hotmail) operate feedback loops you can enrol in; Gmail exposes aggregate complaint data through Postmaster Tools rather than per-message.
• Why it matters - a complainer who stays on your list keeps hurting your reputation; the loop lets you suppress them automatically.
• Who handles it - if you send through a newsletter platform, it is usually enrolled in the major loops on your behalf and acts on complaints for you.

Feedback loops turn invisible spam complaints into a list you can act on - suppress every complainer the moment you learn of them.