Complaint rate is measured as spam reports divided by messages delivered (not sent), usually tracked per campaign and rolled up per domain and IP over a trailing window. It's collected through feedback loops with participating mailbox providers.
Providers generally consider a complaint rate above roughly 0.1% a warning sign and above roughly 0.3% a serious problem likely to trigger filtering, though exact thresholds are not published and vary by provider and sending history.
The most reliable way to keep complaint rate low is making unsubscribe genuinely easy to find and immediate to act on; recipients who can't find an unsubscribe link tend to complain instead.