Complaint
When a recipient hits "Spam" in their mail client — that is a complaint, fed back to the ESP.
A complaint (or abuse report) is an event triggered when a recipient hits "Spam" / "This is spam" in Gmail, Outlook, Yandex, etc.
These complaints are fed back to the sender via the Feedback Loop (FBL) — a formal channel that ESPs need to subscribe to. Postal as the MTA automatically subscribes to the FBLs of major mail services.
A high complaint rate (≥0.1%) is a serious signal to blocklist providers. Sendersy automatically adds complainers to the suppression list and blocks further sends.
The best defense against complaints is sending only to opted-in recipients (double opt-in), using a recognizable From, and shipping a working List-Unsubscribe so people opt out rather than hit Spam.