Metrics
What is Complaint Rate?
The percentage of delivered emails that recipients mark as spam over a given period.
Complaint rate is the number of spam complaints divided by the number of emails delivered, expressed as a percentage. It is typically measured through mailbox-provider feedback loops and postmaster tools, with Gmail Postmaster Tools and Microsoft SNDS being common sources. Mailbox providers generally expect complaint rates to stay below 0.1%, and treat 0.3% and above as a serious problem.
Why it matters for cold email
Complaint rate is the single most actionable reputation metric for cold senders because it crosses provider thresholds quickly and directly drives spam-foldering. Keeping it under 0.1% usually means warming up slowly, scrubbing lists, and pulling non-responders before they get annoyed.
Related terms
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