The 421 4.7.28 error means Gmail has temporarily rate-limited your IP address — too many messages sent too quickly, triggering spam protection mechanisms.
Fix it by pausing email sends for 24-48 hours, verifying SPF/DKIM/DMARC authentication, cleaning your email list, and resuming at reduced volume.
Rate limiting differs from reputation blocks — Gmail isn’t saying your mail is spam, just that you’re sending too much too fast.
The fix requires patience (waiting) combined with behavioral changes (sending less). Attempting to bypass limits by switching IPs often backfires, as Gmail correlates sending infrastructure.
Quick skim — 421 4.7.28 error overview
The 421 4.7.28 error indicates volume-based throttling rather than content rejection.
| Attribute | Details |
| Error code | 421 4.7.28 |
| Category | Rate limiting / throttling |
| Meaning | IP sending unusual volume; temporarily blocked |
| Severity | Temporary (typically 24-48 hours) |
| Common causes | Volume spike, poor reputation, shared IP abuse |
| Fix approach | Pause → verify auth → clean list → resume slowly |
What does IP rate limited mean?
Gmail tracks sending patterns per IP address. When volume or velocity exceeds expected thresholds (relative to your IP’s history and reputation), Gmail throttles delivery to protect recipients from potential spam floods.
Let’s look at the difference between rate limit and domain reputation block:
| Type | Cause | Recovery |
| Rate limit (4.7.28) | Too much too fast | Wait + slow down |
| Reputation block (5.7.1) | Content/behavior flagged as spam | Fix practices + rebuild trust |
Rate limits are easier to recover from — Gmail just wants you to slow down. Reputation blocks require behavioral change and sustained good practices.
But why does Gmail limit the rate? Gmail protects against:
- Shared IP abuse affecting other tenants
- Compromised accounts sending spam bursts
- Legitimate senders accidentally overwhelming infrastructure
- Misconfigured servers flooding recipients
Why does this error occur?
Several patterns trigger Gmail’s rate limiting algorithms.
Volume spike
Sudden increases in sending volume trigger alarms:
- Normal: 100 emails/day → Today: 10,000 emails
- New campaign launch without warmup
- Batch processing accumulated queue
- Misconfigured automation sending loops
Poor IP reputation
Low-reputation IPs face stricter limits:
- PSpam trap hits
- High bounce rates
- Authentication failures
- revious spam complaints
Shared IP contamination
On shared sending infrastructure (common with ESPs):
- IP receives complaints you didn’t cause
- Another tenant’s spam affects your reputation
- Provider doesn’t isolate bad actors quickly enough
Third-party compromise
If using APIs or SMTP relay:
- Hacked accounts in your system
- Compromised API keys sending spam
- Malicious actors using your infrastructure
How do you fix 421 4.7.28?
Immediate response plus long-term changes prevent recurrence.
Pause sending
Stop all non-essential email immediately:
- Halt marketing campaigns
- Pause automated sequences
- Continue only critical transactional mail
- Wait 24-48 hours minimum
Continued sending while rate-limited extends the block duration.
Verify authentication
Confirm proper configuration (rate limiting often accompanies auth issues):
- Verify DKIM signing is active
- Confirm DMARC alignment passes
- Check SPF includes all sending IPs
Run an email deliverability test to verify authentication status.
Check IP reputation
Use Google Postmaster Tools to assess current standing:
- Check for spam rate spikes
- Review IP reputation graphs
- Register your sending domain
- Monitor authentication pass rates
Additionally, check blacklist status across major databases.
Clean your list
Poor list quality contributes to rate-limiting triggers:
- Remove addresses that bounced
- Eliminate purchased or scraped addresses
- Suppress unengaged recipients (no opens in 6+ months)
- Verify addresses before sending with an email validation API
Resume gradually
After the waiting period:
- Start at 10-20% of normal volume
- Increase slowly over 1-2 weeks
- Monitor for 4xx responses
- Pause immediately if rate limiting recurs
Address shared IP issues
If using shared sending infrastructure:
- Contact your ESP about the rate limiting
- Request dedicated IP (if volume justifies)
- Ask about IP isolation and reputation management
- Consider switching providers if issues persist
How do you prevent future rate limiting?
Sustainable sending practices prevent triggering Gmail’s throttles.
Maintain consistent volume
Avoid dramatic spikes:
- Spread large sends across days
- Use scheduled sending for campaigns
- Implement warmup for new infrastructure
Monitor continuously
Watch for early warning signs:
- Track 4xx deferral rates
- Monitor Postmaster Tools weekly
- Set alerts for bounce rate increases
Separate mail streams
Use different IPs or subdomains for:
- Cold outreach
- Transactional mail (receipts, alerts)
- Marketing mail (newsletters, promotions)
Reputation problems in one stream won’t contaminate others.
Still stuck after trying the fix?
Some email errors are easy to clear. Others point to deeper deliverability issues involving authentication, sender reputation, blacklisting, routing, or mailbox provider policy. If you would rather have an expert review it, speak with an email delieverability consultant for free and we can help diagnose the issue and fix it on your behalf.
We look beyond the error message itself to find what is actually breaking delivery, trust, or inbox placement.
From SPF, DKIM, and DMARC to blacklist cleanup, DNS alignment, and sending setup, we can guide or implement the fix.
We assess whether the error is part of a bigger pattern hurting opens, replies, and overall campaign performance.
Talk to a real deliverability expert, get honest guidance, and see the next best step without pressure or upsells.
When should you book a consultation? If the error keeps coming back, affects multiple mailboxes or domains, started after an ESP or DNS change, or is tied to spam placement, low inboxing, high bounce rates, or authentication failures, it is usually faster to get an expert involved early.
Frequently asked questions
Here are some commonly asked questions about this error:
Typically, 24-48 hours if you stop sending. Continued attempts during the block extend the duration. After the limit lifts, resume gradually — jumping back to full volume may trigger re-limiting.
Rarely works and often backfires. Gmail correlates sending infrastructure (domain, authentication, content patterns). Switching IPs while exhibiting the same behavior looks like deliberate evasion — a spam signal that worsens reputation.
Yes. The 4.7.28 code specifically indicates rate limiting (volume concern). The 4.7.0 code is broader — covering suspicious content, authentication issues, and general policy concerns. Both are temporary, but fixes differ: 4.7.28 requires slowing down, while 4.7.0 often requires authentication or content fixes.
