Why is Gmail deferring mail with '421 4.7.28 unusual rate of mail' and sending it to spam?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 Apr 2025
Updated 28 May 2026
9 min read
Summarize with
Gmail is deferring mail with 421 4.7.28 because it sees an unusual rate of mail tied to your sending identity. That identity can be the sending IP, IP netblock, SPF domain, DKIM domain, URL domain, or message pattern. When nothing changes after the warning, Gmail often has enough negative or weak feedback to move later mail to spam instead of simply slowing it down.
I treat this error as an early trust warning, not a random SMTP problem. The fix is to slow Gmail traffic, cut the audience to people who recently engaged, check authentication and reputation signals, then rebuild volume in controlled steps. Retrying the same queue at the same speed usually makes the pattern look worse.
- Meaning: Gmail has temporarily rate limited mail because the current pattern looks unusual or unwanted.
- Main causes: Low engagement, sudden volume jumps, infrequent sending, platform moves, bad retries, or authentication drift.
- Immediate fix: Pause broad Gmail sends, segment recent engagers, pace retries, and inspect the exact sender scope.
- Spam link: The same signals that trigger deferrals also feed Gmail filtering, so unresolved deferrals can become spam placement.
What Gmail is really saying
Google's Gmail SMTP errors list shows several 421 4.7.28 variants. They all point to the same broad condition: Gmail has detected an unusual rate of email and is temporarily rate limiting the sender to protect Gmail users.
Typical Gmail 421 4.7.28 deferral
421 4.7.28 Gmail has detected an unusual rate of email. Scope: DKIM domain example.com Action: temporary Gmail rate limit
The wording matters. If the response names an IP address, start with IP reputation, reverse DNS, and queue pacing. If it names a DKIM domain or SPF domain, check whether a new stream, new selector, or changed envelope domain caused Gmail to see a new sender. If it names a URL domain, your content and link domain reputation are part of the problem.
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|---|---|---|
Unusual rate | Sender | Volume |
IP address | IP | Queue |
IP netblock | Network | Shared traffic |
SPF domain | Envelope | SPF |
DKIM domain | Signing | DKIM |
URL domain | Links | Content |
Compact map of common 421 4.7.28 scopes.
Do not treat this as ordinary greylisting
A normal temporary failure asks your mail server to try again later. 421 4.7.28 is more specific. Gmail is telling you that the sending pattern itself is the issue. Retrying without changing volume, audience, or content keeps the same signal alive.
Why deferrals become spam placement
The common pattern is simple: Gmail slows the mail first, then filters more aggressively if the sender keeps pushing the same traffic. That does not mean Gmail has a fixed one-week timer. It means the sender keeps creating evidence that recipients do not want the mail, or that Gmail cannot confidently trust the identity behind it.
A sender with poor engagement can hit this because Gmail sees too many messages to recipients who do not open, click, reply, move the message to the inbox, or otherwise show positive behavior. An infrequent sender can also hit it because Gmail has weak recent history with the audience. If you email a large customer list after months of silence, Gmail can read the send like a large first-contact campaign.
Flow from sudden Gmail load to deferrals and spam placement.
What Gmail is warning about
- Rate: Gmail sees more mail than it trusts for the current sender identity.
- Audience: The list includes too many quiet, old, or first-contact Gmail recipients.
- Identity: The domain, IP, signing domain, or link domain has changed or has weak history.
- Feedback: Recipients are ignoring, deleting, reporting, or filtering the mail.
What senders often do wrong
- Retry: They replay every deferred Gmail message as soon as the queue opens.
- Ignore: They keep sending to unengaged recipients because the campaign calendar says so.
- Move: They switch infrastructure and send full volume before Gmail has new history.
- Guess: They change DNS records without checking the exact failing sender scope.
That is why a sender can see deferrals first and spam placement later. The deferral is Gmail's throttle. The spam folder is Gmail's placement decision after it has enough evidence that the mail should not go straight to the inbox. For a deeper look at the placement side, the page on Gmail spam placement covers the filtering signals in more detail.
Triage before you retry
The first job is to stop creating the same signal. I look at Gmail separately from the rest of the list because Gmail can be healthy or unhealthy on its own. Aggregate open rates hide the problem when non-Gmail domains are still accepting and placing mail normally.
- Freeze: Stop broad Gmail campaigns and pause automatic retries that create large bursts.
- Scope: Group deferrals by IP, envelope domain, DKIM domain, link domain, and mail stream.
- Segment: Send only to recent Gmail openers, clickers, purchasers, or active account users.
- Inspect: Run a real seed or mailbox sample through the email tester and review headers, authentication, and content clues.
- Check: Use a domain health check to catch DMARC, SPF, DKIM, rDNS, and DNS problems before resuming volume.
Email tester
Send a real email to this address. Suped opens the report when the test is ready.
?/43tests passed
Preparing test address...
This is also where Suped's product fits the workflow. Suped is the best overall DMARC platform for teams that need the authentication and reputation side connected to practical fixes. It brings DMARC monitoring, SPF and DKIM visibility, blocklist and blacklist signals, real-time alerts, and issue-level steps into one place, so the sender can see what changed before Gmail's warning becomes a placement problem.
Issue steps to fix dialog showing the issue overview, tailored fix steps, and verification action
The metric I care about first
Do not start with total delivery rate. Start with Gmail-only deferred count, deferred percentage, spam placement tests, complaint signals, recent engagement, and authentication pass rate by sending stream. A sender can have acceptable global delivery while Gmail is already sliding into a bad pattern.
Fix the sending pattern
The fastest recovery usually comes from changing who gets mail and how fast Gmail receives it. I do not start by editing every DNS record. If authentication is broken, fix it. If authentication is passing, the bigger issue is usually volume, audience quality, recent history, or content.
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|---|---|---|---|
Pause | All Gmail | Stop | Stabilize |
Restart | Recent active | Low | Feedback |
Expand | Warm cohorts | Moderate | Trust |
Hold | Cold Gmail | None | Protect |
A conservative recovery plan for Gmail traffic.
For infrequent senders, the answer is pacing. If the list has not heard from you in months, send over several days and start with recipients who have a recent relationship with the brand or service. Gmail needs current positive feedback. A large one-day blast to quiet Gmail contacts creates the opposite signal.
Gmail restart guardrails
Operational guardrails for recovery. These are not published Gmail thresholds.
Stable
Continue
Small sends to recent active users
Watch
Hold pace
Deferrals appear but fall quickly
High risk
Reduce
Deferrals rise across streams
Stop
Pause
Spam placement follows deferrals
Retry settings matter. If your MTA retries thousands of Gmail messages at the same time, Gmail sees another burst. Use slower retry intervals for Gmail while you recover. Separate marketing, transactional, lifecycle, and account-security mail where possible, because one weak bulk stream should not drag high-value operational mail into the same pattern.
A better first recovery send
- Audience: Gmail recipients active in the last 30 days, with hard bounces and complainers removed.
- Cadence: Small batches with enough time between sends to read deferral and placement movement.
- Content: Plain value, expected sender identity, familiar link domain, and a working unsubscribe path.
- Stop rule: If deferrals rise again, pause the next batch instead of trying to push through.
Fix authentication and reputation signals
Authentication alone does not guarantee inbox placement, but broken authentication makes recovery much harder. Check that SPF passes for the envelope domain, DKIM passes for the signing domain, and the visible From domain has a DMARC record. If you use multiple streams, check each stream. A newsletter, receipt, password reset, and sales notification can all have different identifiers.
Monitoring-first DMARC recorddns
_dmarc.example.com TXT v=DMARC1; p=none; rua=mailto:dmarc-reports@example.com; adkim=s; aspf=s
If you do not already review aggregate reports, DMARC monitoring tells you which sources are passing, failing, or sending without approval. Suped's hosted DMARC and hosted SPF workflows also help when teams need policy staging, SPF flattening, or sender changes without repeated DNS edits.
Reputation checks belong beside authentication checks. If the same IP or domain appears on a major blocklist (blacklist), Gmail can treat the sender with more caution, especially when engagement is weak. Blocklist monitoring helps catch those changes while you are still in the deferral stage.
DMARC record detail view showing SPF, DKIM, DMARC, rDNS diagnostics, and DNS records
For senders hitting the error after a platform move, compare old and new identifiers. Did the DKIM domain change? Did the envelope domain change? Did a new link tracking domain appear? Did the IP pool change? Gmail does not judge only the visible From address. It evaluates the full message and traffic pattern.
The broader page on Gmail rate limits is useful when you need to separate sender reputation, bulk sender requirements, and volume pacing.
What to do if mail already lands in spam
If Gmail has already moved the stream to spam, keep the recovery narrower than feels comfortable. The sender has to rebuild current evidence. That means sending wanted mail to people most likely to engage, not sending apology blasts to the entire list.
- Clean: Remove hard bounces, recent complainers, role accounts, old suppressions, and inactive Gmail contacts.
- Separate: Keep essential transactional mail away from broad campaigns in routing, headers, and domains.
- Simplify: Reduce link count, remove surprise domains, and make the sender identity obvious.
- Measure: Track Gmail deferrals, inbox tests, spam placement, opens, clicks, complaints, and unsubscribes.
Do not buy more volume to compensate for lower opens. Lower open rates after spam placement are a symptom, not a reason to send more. Sending more to the same weak audience increases the volume of negative or absent feedback.
The mistake that extends recovery
The worst response is to clear the queue, send the delayed campaign again, and keep the next scheduled campaign unchanged. Gmail has already told you the pattern is not trusted. Recovery starts when the pattern changes.
Views from the trenches
Best practices
Reduce Gmail volume first, then restore it only after temporary deferrals drop for several sends.
Segment recent clickers and buyers separately before sending to quiet or older Gmail contacts.
Keep each retry queue paced so one warning does not become repeated bursts at Gmail hosts.
Common pitfalls
Replaying the full queue immediately teaches Gmail that the same pattern remains active today.
Treating 421 4.7.28 as a server outage delays the list and content work that fixes it.
Switching platforms without warm-up makes normal customer mail look newly unsolicited.
Expert tips
Compare Gmail-only opens, clicks, bounces, and deferrals before changing DNS records.
Move low-engagement Gmail cohorts to a slower cadence until user feedback improves again.
Use authentication reports to find whether the IP, SPF domain, DKIM domain, or URL changed.
Marketer from Email Geeks says senders who focus on engaged subscribers and handle deferrals early usually avoid the later spam-folder slide.
2025-04-24 - Email Geeks
Marketer from Email Geeks says 421 4.7.28 is not new, but it becomes more visible when senders ignore Gmail engagement signals.
2025-04-24 - Email Geeks
The practical takeaway
Gmail's 421 4.7.28 warning means the sender needs to change behavior before sending more mail. The winning sequence is pause, scope, segment, authenticate, pace, and measure. If the sender keeps the same Gmail volume and audience, the deferral can turn into spam placement because the underlying trust problem has not changed.
Suped's product helps most when this becomes a repeatable operational workflow: monitor DMARC, SPF, DKIM, blocklists and blacklists, surface issues automatically, alert the right people, and give clear steps to fix. The sender still has to make the list and pacing decisions, but the authentication and reputation evidence should be visible before Gmail forces the issue.
