Deliverability & the inbox
Complaint rate
Also known as: spam complaint rate · SCR
The percentage of sent messages that a recipient marks as spam — the single most damaging deliverability signal. Gmail specifically enforces a 0.3% threshold; above that, delivery is systematically degraded for weeks.
Complaint rate is the percentage of sent emails that recipients mark as spam via their mailbox provider's "report spam" button. Every complaint is a strong negative signal — it's the clearest declaration from a recipient that they do not want your mail. Gmail publishes a 0.3% threshold beyond which reputation is materially damaged; operator-consensus safe ceiling is 0.1% (one complaint per 1,000 sends). Complaint rate is tracked per-campaign and in rolling 30-day aggregates. Spikes are usually from sends to a purchased / scraped list, aggressive frequency on an under-segmented audience, or a missing / broken unsubscribe link (users can't find the unsubscribe so they hit "report spam" instead). A campaign exceeding 0.3% complaint rate should trigger immediate pause + audit of the list + review of the last 30 days of sending.