Gmail
Gmail 421 4.7.28 — "unusual rate of unsolicited email" rate limiting
421 4.7.28
Gmail has detected an unusual rate of email, often unsolicited email, from something tied to your messages and is temporarily rate-limiting it. The text says what was flagged: your IP address, your IP netblock, your DKIM domain, your SPF domain, a URL domain in the message, or, in one version, too many messages sharing the same Message-ID. As a 4xx code it's temporary: Gmail is asking you to slow down and retry, and well-behaved mail servers queue and retry automatically.
How to fix it
- 1
Read which identifier Gmail flagged
"from your IP address" or "IP Netblock" points to the sending infrastructure; "DKIM domain" or "SPF domain" points to your domain's reputation; "containing one of your URL domains" points to a link in the message, which can be a shared link shortener or tracking domain; "same Message-ID" means your system is reusing Message-IDs.
- 2
Reduce the sending rate and let retries work
Don't push harder or switch IPs to get around the limit. Lower the send rate to Gmail recipients, keep retrying with backoff, and spread large campaigns over a longer window. New domains and IPs need gradual warmup.
- 3
Lower complaints
Rate limiting follows recipients marking mail as spam. Check your spam rate in Google Postmaster Tools: Google asks bulk senders to stay below 0.3% and recommends staying under 0.1%. Remove unengaged and purchased addresses, make unsubscribing easy (one-click for bulk mail), and only send what people signed up for.
- 4
Make sure authentication is complete
Gmail's bulk sender rules require SPF, DKIM and a DMARC record, with the From domain aligned. Unauthenticated mail gets stricter treatment. Run the email health check on your domain.
- 5
Fix duplicate Message-IDs
If the bounce mentions the same Message-ID, your sending code or platform is reusing an ID across many messages. Every message needs a unique Message-ID header.
Meaning current as of 2026-10-24, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.