550: Generic Delivery Failure

The 550 SMTP error is the most common hard bounce code and can mean many things. Learn how to read the diagnostic text to find the specific cause.

Published · Last verified · Maintained by TamingDNS

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Permanent Failure (Hard Bounce)
SMTP Code
550
Category
Generic Delivery Failure
Frequency
Common

💬 What This Error Means

The door is locked and the person isn't home. This is the most common 'hard bounce' code. It usually means the mailbox doesn't exist, but it's often accompanied by a more specific code (like 5.1.1) that tells you exactly why the door is locked.

Common Causes

  • The email address is spelled wrong (typo)
  • The account has been deleted or disabled
  • Your authentication (SPF/DKIM) failed
  • Your IP or domain is on a blocklist

How to Fix This

  • Look for a more specific code (like 5.x.x) in the bounce message for a better explanation
  • Check the address for typos
  • Use our Domain Checker to verify your authentication setup
  • Use our Blacklist Checker to see if your IP is being blocked

📚 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. 

550 Requested action not taken: mailbox unavailable
550 No such user here

🔗 Related Error Codes

550 5.1.1
Mailbox Does Not Exist
550 5.7.1
Message Rejected by Policy
550 5.7.26
DMARC Policy Violation
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