Gmail 421 4.7.26
Rate limited: unauthenticated
Gmail 421 4.7.26 means: rate limited: unauthenticated. It is a temporary failure, so your server retries it. Gmail wants every sender to pass SPF or DKIM. Publish SPF for the sending IP and sign with DKIM on your own domain, then let the queue retry.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
This email 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 domain-name with ip: ip-address = did not pass. To resolve this issue, go to Email sender guidelines.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | 4.7.26: Multiple authentication checks failed |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
In practice
The temporary version of 5.7.26: Gmail rate-limits unauthenticated mail before it starts blocking it. The bounce includes the DKIM and SPF results Gmail saw. Messages already queued will retry, but they will keep failing until SPF or DKIM passes for the sending domain. Add your sending service to SPF and publish its DKIM key.
What the sender should do
Gmail wants every sender to pass SPF or DKIM. Publish SPF for the sending IP and sign with DKIM on your own domain, then let the queue retry.
Would verifying the list have prevented Gmail 421 4.7.26?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Gmail 421 4.7.26 mean?
- Gmail 421 4.7.26 means: rate limited: unauthenticated. It is a temporary failure, so your server retries it. Gmail wants every sender to pass SPF or DKIM. Publish SPF for the sending IP and sign with DKIM on your own domain, then let the queue retry.
- Should I retry after Gmail 421 4.7.26?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 421 4.7.26?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
The same code elsewhere
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05