4.7.27

Enhanced Status Code 4.7.27: Rate Limited - SPF Authentication Failed

Temporary failure High severity Authentication RFC 7505
What it means

Enhanced Status Code 4.7.27 means “Rate Limited - SPF Authentication Failed.” Google is throttling your mail specifically because SPF did not pass. The message is deferred rather than refused, but the throttle stays until the SPF failure is fixed.

At a glance
Code4.7.27
Bounce typeSoft (temporary)
SeverityHigh
CategoryAuthentication
What to doQueued and retried automatically
StandardRFC 7505
What it looks like in your mail logs 
421 4.7.27 Email rate limited. SPF authentication failed for the sending domain

What does 4.7.27 mean?

In practice 4.7.27 is a Google code. It is returned with a 421 reply and text saying the message is rate limited because SPF authentication failed. The registry assigns X.7.27 to a sender address with a null MX, but a null MX is a permanent fact about DNS rather than a temporary condition, so the 4.x form of that meaning is not something a receiver has cause to send. Google's usage is the one senders meet.

The distinction from a plain SPF rejection matters. Google is not refusing the message, it is slowing you down and telling you exactly why. That makes 4.7.27 one of the more useful bounces you can receive: it names a single mechanism, so there is no ambiguity about whether the problem is DKIM, alignment, content or reputation. SPF did not pass, and Google has decided that unauthenticated mail from you gets a smaller share of its capacity.

Because it is a deferral, mail keeps flowing at a reduced rate and the problem stays invisible on delivery dashboards that only count final outcomes. The tell is throughput: sending takes far longer to drain than it should, with no permanent failures to explain it. Check your SPF result before assuming a capacity problem at your own end.

How 4.7.27 plays out

Your server attempts delivery
The recipient defers with a temporary 4.7.27 reply
Your server queues the message and retries on a back-off schedule
It delivers on a later attempt, or becomes a hard bounce if it keeps failing

Where 4.7.27 sits: soft vs hard bounce

Soft bounce (4xx) Hard bounce (5xx)
NatureTemporaryPermanent
SMTP class4xx5xx
What to doLet it retrySuppress the address
Recoverable?OftenNo
4.7.27 is✓ this code

What each provider means by 4.7.27

A registered code does not oblige a provider to use it that way, and the large ones diverge. Match the wording in your own bounce, not the definition above.

Gmail Email rate limited because SPF authentication failed Reference
IANA registry The registry assigns 4.7.27 to a sender address with a null MX Reference

Common causes of 4.7.27

  • The sending IP address is not authorised in the SPF record for the envelope sender domain
  • The SPF record exceeds the ten DNS lookup limit and evaluation returns permerror
  • The domain publishes two SPF records, which is invalid and fails outright
  • A new sending platform was added without being included in the record
  • The envelope sender uses a subdomain that has no SPF record of its own
  • Mail is being forwarded, so the connecting IP is a forwarder rather than yours

How to fix 4.7.27

  • Check the SPF record and its lookup count with SPF Record Checker
  • Send a test message and confirm spf=pass appears in the Authentication-Results header
  • Add every sending platform, and publish a record for any subdomain used as an envelope sender
  • Consolidate to exactly one SPF record for the domain
  • Sign with DKIM as well, since DKIM survives forwarding where SPF cannot
  • Expect throughput rather than bounce rate to be the visible symptom, and monitor it

Frequently asked questions

What does 421 4.7.27 mean?
From Google it means your mail is being rate limited because SPF authentication failed. It is a temporary deferral, so messages are retried and generally do arrive, but at a reduced rate that persists until SPF passes. The code names one mechanism only, which makes the diagnosis unusually clear: fix SPF and the throttle lifts.
Why does the registry say 4.7.27 means null MX?
The IANA registry assigns X.7.27 to a sender address with a null MX record, and that meaning is real for the permanent form, 5.7.27. It does not translate to a temporary code, because a null MX is a fixed fact about a domain rather than a passing condition. Google occupied the 4.x form for SPF rate limiting and that is what senders actually receive.
How is 4.7.27 different from 4.7.26?
They differ in how much they tell you. 4.7.26 means more than one authentication check failed, so SPF and DKIM are both suspect. 4.7.27 names SPF specifically, which means DKIM may well be passing. If you see 4.7.27, the investigation is confined to your SPF record and the IP addresses it authorises.
My mail still arrives, so does this matter?
Yes. A deferral that resolves on retry hides the problem from any dashboard that only counts final delivery, while your effective sending capacity to Google is quietly reduced. The visible symptom is a queue that drains far more slowly than it should. Left alone, a sender in this state is one policy change away from permanent rejection.
Will fixing SPF alone clear this?
It clears this specific code, but SPF alone is no longer sufficient for bulk senders. Since February 2024 Google has expected both SPF and DKIM from senders at volume, and DKIM is the mechanism that survives forwarding. Fix SPF to lift the throttle, then confirm DKIM is signing and aligning so you are not back here under a different code.

Related bounce codes

5.7.23 SPF Validation Failed 4.7.26 Multiple Authentication Checks Failed - Temporary 4.7.30 Rate Limited - DKIM Authentication Failed 5.7.24 SPF Validation Error 4.7.28 Mail Flood Detected - Temporary Rate Limit

Sources & further reading

RFC 7505 · The standard that defines this reply code Gmail SMTP errors and codes · Google Workspace reference for every code Gmail returns IANA registry · The authoritative list of enhanced status codes and their meanings
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes