421: Service Temporarily Unavailable

The 421 SMTP error means the receiving server is temporarily unavailable or closing the connection. Learn what causes 421 errors and when to retry.

Published · Last verified · Maintained by TamingDNS

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Temporary Failure (Soft Bounce)
SMTP Code
421
Category
Service Temporarily Unavailable
Frequency
Common

💬 What This Error Means

The receiving server is temporarily busy or 'closed for maintenance.' This is a very common soft bounce that means the system just can't handle your message at this exact second. Your server will try again soon.

Common Causes

  • The server is rebooting or installing updates
  • There are too many connections at once
  • A brief network glitch interrupted the connection

How to Fix This

  • No action is needed. Your server will automatically try again
  • If you're sending a large newsletter, try slowing down your sending speed

📚 Official Documentation

↗ RFC 5321: SMTP

📋 Real-World Example Messages

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

421 Service temporarily unavailable
421 4.3.2 Service temporarily unavailable

🔗 Related Error Codes

421 4.3.2
System Not Accepting Mail
421 4.4.1
No Answer from Host
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