Complaint rate is the metric to watch, and the thresholds are far lower than most people expect.
0.3% is the ceiling Google publishes in its sender guidelines. 0.1% is what Google actually tells you to stay below in its own troubleshooting guidance. Yahoo uses 0.3% as well, calculated on mail delivered to the inbox.
To put that in proportion: on a send of 100,000, thirty complaints is a warning and one hundred is a serious problem. It takes remarkably few annoyed people.
What you never see
Not every complaint reaches you. Feedback loops exist for some providers and not others, and even where they do, the data is partial. Gmail in particular reports aggregate spam rate through Postmaster Tools but does not hand you the individual complaints.
So the rate you can measure is a floor, not a total.
The causes, in order of how often they are the actual cause
- Frequency. Fatigue is the most common driver and the least dramatic. People who liked you in January mark you as spam in March because you mailed them forty times.
- Unexpected mail. The recipient does not recognise you, usually because the signup was months ago or the sender name does not match the brand they remember.
- A hard-to-find unsubscribe. If opting out looks like work, marking spam is the faster option — and it costs you far more.
- Content genuinely not matching the promise made at signup.
Note that only the last one is about the email itself. Three of the four are operational.
The multi-brand angle
Complaint rate is measured against the sending domain and IP, so brands sharing infrastructure pool their complaints. A portfolio where one brand mails aggressively and four are careful produces a blended rate that punishes all five.
Two mechanisms contain it: separate sending domains per brand, and frequency caps that are enforced across campaigns and automations rather than per campaign. The second matters more than people expect, because over-mailing is almost never a decision — it is the compound effect of a campaign, a lifecycle flow and an announcement all looking reasonable on their own.