A spam complaint is a signal from a mailbox provider that a recipient marked one of your messages as spam. The complaint rate is the ratio of these to the volume delivered to that provider's inbox. It is the single most negative signal in modern deliverability.
Thresholds
Gmail's stated threshold is 0.3%. Above it, aggressive filtering is expected. Practical target for a healthy programme is under 0.1%; excellent programmes sit under 0.05%. Complaint rates above 0.5% cause reputation collapse within days.
Where complaints come from
- Unclear or missing opt-in — recipients do not remember signing up
- Frequency too high — the recipient consented once but did not consent to daily emails
- Content mismatch — signed up for one thing, receiving another
- Unsubscribe friction — clicking spam is easier than finding unsubscribe
Structural fixes
Adopt one-click list-unsubscribe (RFC 8058). Reduce frequency where recent engagement is low. Segment aggressively; re-engagement campaigns to stale segments produce complaints out of all proportion to their volume. Suppress non-openers over 90 days from broadcast streams. Never resurrect a suppressed address.
Related articles
- SPF ExplainedHow Sender Policy Framework works, how to author a record correctly, and how to avoid the common failure modes.
- DKIM ExplainedDomainKeys Identified Mail — how signing works, selector strategy, key length and rotation, and what DKIM cannot do.
- DMARC GuideHow DMARC ties SPF and DKIM together, staged policy rollout from monitor to reject, and how to use aggregate reports.
- Google Postmaster GuideHow to set up Postmaster Tools, what each metric means, and how to interpret the reputation graph.
- PowerMTA OptimizationVMTA design, queue tuning, per-domain policy, and the configuration mistakes that quietly cost inbox rate.
- IP Warm-upHow to warm a new IP without wasting weeks or torching reputation. Volume curves, segmentation, and when to stop.