/ Glossary — Metrics
What is complaint rate?
The share of recipients who hit 'mark as spam' — the single most damaging signal you can generate, and one mailbox providers now enforce a hard cap on. Here's the number that matters.
/ Definition
Complaint rate, sometimes called spam rate, is the percentage of recipients who mark your mail as spam. It's among the most damaging signals in email, because a complaint is a direct statement from a recipient that they didn't want your mail — and mailbox providers weight it heavily when deciding your sender reputation.
The numbers are unforgiving. Google and Yahoo's bulk sender rules require keeping the complaint rate below 0.3%, and strongly advise staying under 0.1% — thresholds so low that even a small share of annoyed recipients can push you over. Complaints are reported to senders through feedback loops, which is how you find out which mail is generating them.
/ In practice
Why complaint rate is so punishing
A complaint carries more weight than almost any other signal because it's unambiguous: the recipient actively chose to report your mail as unwanted. Mailbox providers treat even a low complaint rate as a serious problem, which is why the enforced thresholds are measured in fractions of a percent. Crossing them doesn't just affect the complained-about campaign — it damages your reputation broadly, lowering inbox placement for all your mail. This is why a single poorly-targeted send to an unengaged segment can do outsized harm.
Keeping complaints low
Complaints come from sending mail people don't want, so the fixes are about consent and relevance: send only to people who opted in, make unsubscribing easy so frustrated recipients leave rather than complain, honor a one-click unsubscribe instantly, and stop mailing unengaged recipients before they turn on you. A sunset policy that retires inactive contacts is one of the most effective complaint-reducers, because the people most likely to complain are those who stopped wanting your mail long ago.
/ Reading the data
Where complaint data comes from
Complaint data reaches you through feedback loops and provider dashboards like Google Postmaster Tools, which report the spam-complaint rate Gmail sees for your domain. Watching these together is how you catch a rising complaint problem while it's still well under the enforced thresholds, rather than discovering it after deliverability has already dropped.
/ Related terms
Complaint rate is measured in fractions of a percent. We keep you under the line.
Consent-based sending, instant unsubscribe handling, and sunset policies keep complaints below the thresholds that matter. Bring us your sending and we'll protect the number.
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