Definition
Complaint rates feed directly into sender reputation at mailbox providers. Even modest rates trigger foldering, throttling and blocklisting. The causes are predictable: bought lists, unclear consent, excessive frequency, missing unsubscribe paths.
Why it matters
One complaint per thousand sends starts hurting; prevention (expectation-setting at signup, easy unsubscribes) beats any remediation.
Formula
(Complaints / Delivered) x 100
Benchmarks
Under 0.1%; act above 0.3%
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