What complaint threshold should I aim for as a sender?
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The threshold you should aim for is lower than the threshold that will get you into trouble. That distinction matters more than people think.
Most mailbox providers tolerate complaint rates up to 0.10% before they start affecting your inbox placement. Google Postmaster Tools publishes this explicitly: above 0.10% consistently and your domain reputation starts to drop; above 0.30% and you're likely being bulk-filtered to spam. Those are the published danger thresholds. But you shouldn't be aiming for the danger threshold. You should be aiming for 0.01% or below, which is roughly one complaint per 10,000 messages.
Here's why the gap matters: complaint rates are volatile. A single poorly targeted campaign, a re-engagement email sent to the wrong segment, or a subject line that sounds spammy to a lot of people can spike your rate for that campaign. If you're normally running at 0.08%, a spike puts you above 0.10%. If you're normally running at 0.01%, a spike to 0.05% isn't even close to the danger zone. The lower your baseline, the more buffer you have for the occasional bad send.
To stay at the low end: make sure your list-unsubscribe header is implemented and one-click compliant so people who want out can get out without hitting the spam button. Segment carefully so you're not sending irrelevant content to people who have no interest in it. And monitor your complaint rate per campaign, not just as a total average. A pattern of spikes on certain send types usually tells you exactly where the problem is.
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