Complaint rate

Bounces say your data is stale. Complaints say the person who received your message did not want it — which is precisely the judgement a spam filter exists to make, delivered by the only party whose opinion is authoritative. Providers weight it accordingly, and recovery is slow.

Note the denominator: complaints are measured against delivered mail. Improving your bounce rate without improving relevance can therefore raise your complaint rate, because more of what you send is arriving in front of people.

Rarely from a single bad message. The common causes are structural:

• Irrelevance at volume — the message is fine and the recipient was never a plausible buyer.
• A hidden or broken unsubscribe, which turns every removal request into a spam report.
• Excessive follow-up. The fourth unanswered message is far more likely to be reported than the first.
• Role addresses and shared mailboxes, read by people with the least patience for unsolicited mail.

At a 0.3% ceiling, three complaints in a thousand delivered messages is the limit. On a fifty-a-day mailbox that is a handful of people over a month. There is no volume at which complaints average out — a higher send rate simply reaches the threshold faster.

Which makes the control obvious: fewer, more relevant messages, an unmistakable way to opt out, and an automatic stop when the rate climbs.

SalesShift pauses a mailbox when its complaint rate reaches 0.3% and at least two complaints have been recorded. Complaints are also the heaviest penalty in the health score — worth up to 40 of the 90 base points, more than bounces or transport failures.