Gmail error 421-4.7.28 "unusual rate of unsolicited mail" — what it means and how to fix it

Short answer: Gmail is rate-limiting you, not blocking you — yet. Error 421-4.7.28 ("Gmail has detected an unusual rate of unsolicited mail originating from your IP/domain") is a temporary deferral: Gmail's filters saw a volume or pattern anomaly and are slowing you down while they evaluate. Keep hammering and it escalates to permanent 550 rejections.

What triggers it

• Volume spikes — especially from young domains or after quiet periods. The classic cold-email ramp mistake.
• Reputation dips — spam-rate creeping toward Google's 0.3% ceiling, or a blacklist listing feeding their models.
• Authentication gaps — SPF/DKIM/DMARC failures or misalignment make all volume look suspicious.
• Shared-IP contamination — a noisy neighbor on your ESP's pool can trigger it for everyone.

Fix sequence (in order)

1. Pause or halve sending to Gmail recipients for 24-48h. The limit is adaptive; continued pressure extends it.
2. Verify authentication end-to-end — grade your domain: SPF within the 10-lookup limit, DKIM valid, DMARC aligned.
3. Check blacklists — a fresh listing often precedes this error. Run the check.
4. Check Google Postmaster Tools for your spam-rate and delivery-error trends (v2 shows verdicts per subdomain).
5. Resume gradually — 50% volume for several days, consistent daily patterns, no bursts.

How long does 421-4.7.28 last?

Typically hours to 2-3 days once the pattern normalizes. If it persists past a week at reduced volume, the underlying reputation problem (not the rate limit) is your real issue.