What is a complaint rate?
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Your complaint rate is the percentage of delivered emails where someone hit the spam button inside their mailbox. It's not about your email going to spam. It's about a person actively telling their mailbox provider that your email shouldn't be there.
The formula: (spam complaints / emails delivered) x 100. Send 5,000 emails, get 3 spam reports, your complaint rate is 0.06%. That sounds tiny, but it's actually a meaningful signal to Gmail, Outlook, and other mailbox providers. They aggregate these signals across all senders and use them to decide how much to trust your domain.
The key difference between a complaint rate and an unsubscribe rate: an unsubscribe affects only your list. A complaint affects your reputation with the mailbox provider. That's why complaint rates are considered the most damaging negative signal a sender can accumulate. Unsubscribes don't damage deliverability directly. Complaints do.
Most ESPs show complaint rate in your campaign reports, but they may only show what's reported back to them through feedback loops. Gmail doesn't report individual complaints to senders. It shows aggregate complaint rate data through Google Postmaster Tools, which is why checking Postmaster Tools regularly is worth doing if Gmail is a significant portion of your audience. For everything on thresholds, see what's an acceptable complaint rate.
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