421 4.7.27 Gmail bounce: spf fail, rate limited
What 421 4.7.27 means
Gmail is rate limiting mail from a sender whose SPF check did not pass.
Message text
Your email has been rate limited because SPF authentication didn't pass…
This is an excerpt: the provider's documentation shortens the message. Full text: provider documentation
Common causes of 421 4.7.27
- SPF check fails for the sending domain or IP address
How to fix 421 4.7.27
- Run SPF checker and confirm the sending IP or service is authorised.
- Set up SPF for every domain you send from.
Check your domain
Check the SPF record of your sending domainRuns in your browser; nothing is uploaded.
Get your domain's deliverability score
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 421 4.7.27
The first part, 421, is the SMTP reply code. Service not available, closing transmission channel: the server is shutting down or refusing the connection for now. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 4.7.27, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: knowledge.workspace.google.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 4.7.0 SMTP: security status
- 421 4.7.0 Gmail: no PTR record
- 451 4.7.23 Gmail: PTR record missing
- 421 4.7.28 Gmail: rate limited
- 4.7.26 Microsoft: ipv6 needs spf or dkim
- 451 4.7.24 Gmail: spf record suspicious
FAQ
What does 421 4.7.27 mean?
Gmail is rate limiting mail from a sender whose SPF check did not pass. This is a temporary failure.
How do I fix 421 4.7.27?
Run SPF checker and confirm the sending IP or service is authorised. Set up SPF for every domain you send from.
Is 421 4.7.27 permanent, and should I retry?
421 4.7.27 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.