Gmail / Google Workspace error guide

Gmail / Google Workspace SMTP 421 4.7.30: Dkim Failed Bulk Sender

Gmail / Google Workspace SMTP 421 4.7.30 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.

Quick summary

Primary code
421 4.7.30
Provider coverage
Gmail / Google Workspace
Primary action
Retry later

How to handle 421 4.7.30

Match the provider, SMTP stage, endpoint, and diagnostic wording first, then use the checks below to decide whether to retry, suppress the recipient, or fix sender setup.

Gmail 421 4.7.30

Sender authentication

Retry later

The provider could not verify that the visible sender domain is authorized. SPF, DKIM, and DMARC alignment usually decide whether this clears.

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
DKIM authenticationdidn?t pass

Start here

  1. 1

    Verify SPF, DKIM, and DMARC for the visible From domain.

  2. 2

    Make sure SPF or DKIM passes and aligns with DMARC for the provider that rejected the message.

Confirm the cause

  1. Check SPF for the visible From domain and make sure it includes the active sending IP or service.
  2. Check DKIM signing for the message and confirm the signing domain aligns with the visible From domain.
  3. Check DMARC policy and alignment; if policy is quarantine or reject, make SPF or DKIM pass with alignment before retrying.

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
  • SPF does not include the active sending IP or service.
  • DKIM is missing, failing, or signing with a domain that does not align.
  • DMARC policy is active, but neither SPF nor DKIM passes with alignment.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampSPF resultDKIM resultDMARC result
Classifier evidence
Specific cause
dkim failed bulk sender
Evidence
official
support.google.com source 1

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