TL;DR
4.7.0 is a temporary, unspecified security-status failure. Diagnose the exact reply and policy or authentication context, then retry with limits; it is not a hard bounce by itself.
What this code means
The transient form of X.7.0 is used when a security-related condition cannot be expressed with a more specific detail, or when policy prevents a fuller disclosure. The code identifies neither the exact mechanism nor an invalid recipient address.
Provider examples
smtp;421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes421 4.7.0 Connection expired, try reconnecting. For more information, go to About SMTP error messages. - gsmtp421 4.7.0 IP not in whitelist for RCPT domain, closing connection. For more information, go to Allowlists, denylists, and approved senders. - gsmtp421 4.7.0 Try again later, closing connection. (command) For more information, go to About SMTP error messages. - gsmtp421 4.7.0 The IP address sending this message does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP. To protect our users from spam, email sent from your IP address has been temporarily rate limited. For more information, go to Email sender guidelines. - gsmtp454 4.7.0 Too many login attempts, please try again later. For more information, go to Add Gmail to another email client. - gsmtpMessage temporarily deferred (eg: TS* Errors) — This is a temporary error and your mail server may automatically re-try sending the email at a later time. Your message may have been deferred due to one or more of the following: Emails from your mail server are generating substantial complaints from Yahoo users; The message contained objectionable content or exhibited characteristics indicative of spam; The IP (x.x.x.x or its subnet, i.e., .255 ) has a poor reputation; We are seeing unusual traffic patterns from your mail servers.421 RP-001 — The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.421 RP-002 — The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed on this connection. Reason for rate limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.421 RP-003 — The mail server IP connecting to Outlook.com server has exceeded the connection limit allowed. Reason for limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp421 4.7.0 me.com Error: too many errors451 4.7.0 Temporary server error. Please try again later. PRX4 NextHop: example.com [example.com]451 4.7.0 Temporary server error. Please try again later. PRX6 NextHop: outlook.com [outlook.com]<beegelee@yahoo.com>: host mta6.am0.yahoodns.net[76.159.240.37] said: 421 4.7.0 [TSS04] Messages from 39.177.48.16 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes (in reply to MAIL FROM command)Technical meaning
RFC 3463 defines X.7.0 as another or undefined security status. The leading 4 makes this instance a persistent transient result: the current attempt failed, while the underlying condition may clear after the relevant security or policy state changes.
Delivery status
Class 4 means retry may be appropriate, not that a retry without diagnosis will work. The code alone does not identify a particular authentication mechanism, rule, owner, or duration.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Keep the complete SMTP response, check the suppression state, and use bounded retries with backoff, jitter, and idempotency. Stop after delivery, a permanent result, a policy stop, or the retry limit; investigate recurring 4.7.0 rather than retrying indefinitely.
Suppression decision
Do not suppress a recipient based on 4.7.0 alone. Make suppression decisions from independent recipient-level permanent evidence or the applicable policy, not from this non-specific temporary status.
Common causes
- The server has a security-related condition but cannot safely or correctly disclose a narrower detail.
- A local policy intentionally returns a general security status instead of the precise rule or mechanism.
- The full reply contains the operational clue; the numeric code alone is intentionally broad.
Diagnostic steps
- Confirm the exact enhanced code and basic 4xx reply in the raw response.
- Record the SMTP stage, time, sending identity, receiving host, and complete diagnostic text.
- Check authentication, TLS, policy, and service logs for a matching event without guessing from the code title.
- Retry only in a bounded way after the relevant context has been inspected.
Actions by owner
Sender
- Provide the unredacted operational context and full response to the sending administrator; avoid repeated manual resends.
Sender administrator
- Correlate the reply with authentication, TLS, and policy logs, then apply bounded retry behavior.
Recipient administrator
- If you operate the rejecting service, identify whether a local security policy can provide a safe, actionable diagnostic.
Provider
- Inspect the affected service and return precise diagnostics where policy permits; do not turn a general temporary signal into a recipient verdict.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
- Yahoo SMTP Error Codes — Yahoo Sender Hub guide to SMTP deferral and rejection codes.
- Outlook.com Postmaster troubleshooting — Official Outlook.com Postmaster page with consumer SMTP error codes.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
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