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Bounce Decoder: 421 4.7.28

SPF, DKIM, DMARC, FCrDNS, Blacklists & Inbox Prediction

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421 4.7.28 — Blocked: High Complaint Rate

The 421 4.7.28 error is Google-specific and means your domain or IP is blocked due to high spam complaint rates. Learn how to fix your sender reputation.

🟡
Temporary Failure (Soft Bounce)
SMTP Code
421
Enhanced Code
4.7.28
Category
Blocked: High Complaint Rate
Frequency
Very Common

🔢 Enhanced Status Code Breakdown: 4.7.28

Component Value Meaning
Class 4 Persistent transient failure (temporary)
Subject 7 Security or policy
Detail 28 Blocked: High Complaint Rate

Per RFC 3463 Enhanced Mail System Status Codes. Class (X) = severity, Subject (Y) = category, Detail (Z) = specific condition.

💬 What This Error Means

Your sending IP or domain has been temporarily blocked due to a high rate of spam complaints from recipients. Google specifically uses this code when too many people mark your email as spam.

Common Causes

  • Recipients marking your emails as spam at a high rate
  • Poor list hygiene — sending to disengaged or cold contacts
  • Sending to purchased, scraped, or harvested email lists

How to Fix This

  • Check your complaint rate in Google Postmaster Tools (target: below 0.10%)
  • Remove unengaged subscribers from your list
  • Ensure you are only sending to people who explicitly opted in
  • Add a clearly visible one-click unsubscribe link to all commercial emails

🏢 Provider-Specific Variations

Google Workspace / Gmail

This is a Google-specific code. Your domain or IP reputation is low due to spam complaints. Google Postmaster Tools shows your complaint rate and domain reputation.

📖 Google Workspace / Gmail documentation →

📋 Real-World Example Messages

These are real bounce message formats you might receive. Paste yours into the Bounce Decoder for instant analysis. 

421-4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your IP address.

🔗 Related Error Codes

421 4.7.0
Temporary Policy Rejection
421 4.7.29
Blocked: Local Policy (New Sender)
450 4.2.1
Rate Limit: Try Again Later

📚 Official Documentation

↗ Google Postmaster Tools ↗ Google Bulk Sender Guidelines
🔍

Got a bounce message to decode?

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