Gmail 421 4.7.28
Rate or reputation
The provider is slowing or deferring traffic. Retrying can work, but repeated deferrals usually mean volume, reputation, or recipient engagement needs review.
Decision
A retry can work, especially after a short wait.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- End of DATA
- Endpoint
- Inbound MX
unusual rateunsolicitedsame Message-IDMessage-IDStart here
- 1
Read the full SMTP diagnostic text, not only the numeric reply code.
- 2
Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.
Confirm the cause
- Review recent sending volume, bounce rate, complaint rate, and list hygiene for this provider.
- Confirm SPF, DKIM, and DMARC alignment so reputation problems are not made worse by authentication failures.
- Resume with lower provider-specific volume only after deferrals stop or the provider accepts test mail.
App handling
- Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
- Slow or pause provider-specific sending if the same deferral repeats across many recipients.
- Escalate to reputation, authentication, or volume review when retries keep failing.
Avoid
- Do not suppress the recipient from a temporary response.
- Do not keep the same retry pace if the deferral repeats across many recipients.
- Do not ignore sender reputation or authentication if this keeps happening.
Common causes
- Volume increased faster than the provider trusts for this sender.
- Recent bounces, complaints, or low engagement damaged sender reputation.
- The provider applied a traffic, content, or sender policy to protect recipients.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampprovider retry countsend volume windowbounce rate
Classifier evidence
- Specific cause
- gmail bulk rate or reputation
- Evidence
- official+observed