TL;DR
Gmail 4.7.26 is a temporary unauthenticated-mail rate limit. In the accepted capture, verify that both SPF and DKIM did not pass from the exact reply and logs, correct the responsible configuration, reduce rate, and retry with backoff; it is not a recipient hard bounce.
What this code means
The accepted Gmail response says it has rate-limited unauthenticated mail and requires senders to authenticate with either SPF or DKIM. The concrete code is published from exact Gmail evidence, not from a confirmed IANA registration for this detailed pattern, and the response does not establish a recipient-address failure.
Provider examples
4.7.26 This mail has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass. SPF example.com with ip: x.x.x.x = did not passTechnical meaning
In the accepted Gmail response, the temporary condition is triggered when its authentication results show both DKIM and SPF did not pass. The response identifies Gmail-specific rate limiting for that attempt; it does not claim that every authentication failure at every receiver behaves the same way.
Delivery status
The leading 4 identifies a temporary class in the enhanced status code. The accepted capture must be kept in its exact form; validate the basic SMTP reply class separately from the live transaction before using it as an operational property. The condition may clear after verified authentication remediation and appropriate pacing.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Keep the raw reply and authentication results, verify the SPF and DKIM configuration actually used by the message, correct only confirmed defects, then use backoff, jitter, idempotency, a reduced pace, and an attempt limit. Stop repeating unchanged sends while both mechanisms fail.
Suppression decision
Do not suppress recipients from 4.7.26 alone. The accepted condition concerns sender authentication and rate behavior, not a recipient mailbox; use independent recipient-level permanent evidence for suppression.
Common causes
- The message's SPF and DKIM authentication paths both fail in the accepted Gmail diagnostic.
- A DNS, signing, alignment, or sending-service change changed the authentication result for the traffic.
- Traffic continues at a pace that does not respect Gmail's temporary backpressure.
Diagnostic steps
- Confirm the exact raw Gmail capture and its authentication-result details; validate the basic SMTP reply class separately from the transaction record.
- Correlate the failed attempt with the From domain, envelope sender, DKIM signing domain, sending IP, and current DNS records.
- Check SPF evaluation and DKIM signature results in the sending system without treating an assumed configuration as proof.
- Correct verified authentication defects, reduce send pace, and run bounded retries.
Actions by owner
Sender
- Stop repeated manual resend attempts and provide the full response, sending domain, and time to the sender administrator.
Sender administrator
- Verify SPF and DKIM for the actual sending path, correct confirmed defects, and pace recovery by affected traffic.
Provider
- Inspect signing, DNS, and sending-IP context, then apply rate control while the authentication issue is remediated.
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.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 7372 — Email Authentication Status Codes — Status codes for SPF, DKIM, DMARC and related authentication results.
- Fix bounced or rejected emails — sender authentication requirement — Gmail Help article, authentication section, for the unauthenticated-mail rate-limit condition (421 4.7.26).
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
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