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Bounce Decoder: 421 4.4.7

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421 4.4.7 — Message Expired in Queue

The 4.4.7 bounce means your email spent too long in the queue without delivery and expired. Learn what causes queue expiry and how to investigate.

🟡
Temporary Failure (Soft Bounce)
SMTP Code
421
Enhanced Code
4.4.7
Category
Message Expired in Queue
Frequency
Very Common

🔢 Enhanced Status Code Breakdown: 4.4.7

Component Value Meaning
Class 4 Persistent transient failure (temporary)
Subject 4 Network and routing
Detail 7 Message Expired in Queue

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

💬 What This Error Means

Your email spent too long in the outbound queue without a successful delivery attempt and timed out. The destination server was unreachable for the entire retry period.

Common Causes

  • Recipient mail server was unreachable for an extended period (24–72 hours)
  • DNS resolution failures during the retry window
  • Network routing issue between servers

How to Fix This

  • Check if the recipient domain has working MX records now
  • Try resending manually — the issue may have resolved
  • If this recurs, investigate the recipient domain health

🏢 Provider-Specific Variations

Microsoft 365 / Outlook

Microsoft 365 uses 4.4.7 when your message cannot be delivered within 24 hours. NDR typically arrives with subject "Delivery delayed".

📖 Microsoft 365 / Outlook documentation →

📋 Real-World Example Messages

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

421 4.4.7 Message expired after maximum queue time
421 4.4.7 QUEUE.Expired; Message expired at

🔗 Related Error Codes

421 4.4.1
No Answer from Host
421 4.4.2
Bad Connection
550 5.4.7
Delivery Time Expired

📚 Official Documentation

↗ Microsoft 365 NDR codes
🔍

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