Error Codes Library – Gmail / Google Workspace

Who This Is For / When to Use

Each error includes:

These errors are usually temporary. Messages may succeed if retried later.

421 4.3.0 – Temporary system problem
- What to do: Retry sending the message later.
421 4.4.5 – Server busy
- What to do: Retry sending later.
421 4.7.0 – IP not whitelisted for recipient domain
- What to do: Verify your sending IP is authorized by the recipient domain.
421 4.7.0 – Unusual rate of unsolicited mail detected
- What to do: Review outbound messages for spam-like behavior.
421 4.7.0 – TLS required
- What to do: Ensure your mail server supports TLS.

450 4.2.1 – Recipient receiving mail too quickly
- What to do: Slow down sending and retry later.
450 4.2.1 – Peak SMTP relay limit exceeded
- What to do: Reduce send volume.

451 4.3.0 – Mail server temporarily rejected the message
- What to do: Retry later.
451 4.4.2 – Connection timeout
- What to do: Retry later.
451 4.5.0 – SMTP protocol violation
- What to do: Verify your SMTP client follows protocol correctly.

452 4.2.2 – Recipient mailbox over quota
- What to do: Ask the recipient to clear storage.
452 4.5.3 – Too many recipients or message too large per transaction
- What to do: Reduce recipients or split sends.

454 4.5.0 – Commands sent after STARTTLS incorrectly
- What to do: Fix SMTP relay configuration.
454 4.7.0 – Temporary authentication problem
- What to do: Retry later or contact your email provider.

These errors usually require configuration or content changes.

501 5.5.2 – Cannot decode response
- What to do: Retry or contact your provider.
501 5.5.4 – Invalid HELO/EHLO argument
- What to do: Fix SMTP server identification settings.
503 5.5.1 – Incorrect SMTP command order
- What to do: Ensure commands start with EHLO/HELO, then MAIL, then RCPT.

530 5.5.1 – Authentication required
- What to do: Verify SMTP username and password.
530 5.7.0 – STARTTLS required
- What to do: Enable STARTTLS before authentication.
535 5.7.1 – Username or password not accepted
- What to do: Use correct credentials or an application-specific password.
535 5.7.1 – Application-specific password required
- What to do: Generate and use an app password if 2-Step Verification is enabled.

550 5.1.1 – Recipient does not exist
- What to do: Check the email address for typos.
550 5.2.1 – Recipient account disabled
- What to do: Contact the recipient via another method.
550 5.4.5 – Daily sending or relay quota exceeded
- What to do: Reduce volume or upgrade Google Workspace limits.
550 5.7.0 – Mail relay denied
- What to do: Verify SMTP relay permissions.
550 5.7.1 – Unauthenticated or spam-like email blocked
- What to do: Ensure SPF, DKIM, and DMARC are configured correctly.
550 5.7.26 – DMARC policy rejection
- What to do: Align SPF/DKIM with the domain’s DMARC policy.

552 5.2.3 – Message exceeds size limits
- What to do: Reduce attachments or message size.
554 5.6.0 – Malformed message or mail loop detected
- What to do: Review headers and routing rules.
555 5.7.1 – Authentication required for relay
- What to do: Provide valid SMTP authentication credentials.

Frequent 4xx errors:
Usually caused by rate limits or temporary Gmail throttling. Slow down sending.
Repeated 5.7.x errors:
Almost always related to authentication, spam policy, or domain configuration.
Quota-related errors:
Check daily send limits, SMTP relay limits, and mailbox storage usage.

4xx errors are temporary and may resolve automatically.
5xx errors are permanent and require configuration or content changes.

Gmail detected spam-like patterns, missing authentication, or policy violations. Review SPF, DKIM, DMARC, and email content.

No. 5xx errors must be fixed before resending.

If authentication, quota, or policy errors persist after correcting settings and reducing send volume.