501: Bad Parameter or Argument

The 501 SMTP error means an invalid parameter was sent to the mail server, often a malformed address. Learn what causes 501 errors and how to fix them.

Published · Last verified · Maintained by TamingDNS

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Permanent Failure (Hard Bounce)
SMTP Code
501
Category
Bad Parameter or Argument
Frequency
Common

💬 What This Error Means

There's a mistake in the 'delivery instructions.' The server received a piece of information it didn't like, most often a malformed email address in the hidden 'envelope' of the message.

Common Causes

  • An invalid character or space in the email address
  • The address is formatted in a way that the server doesn't understand

How to Fix This

  • Check the recipient's email address very carefully for any small typos
  • If the address looks perfect, the problem might be a hidden setting in your email software

📚 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. 

501 Bad address syntax
501 5.1.3 Invalid address format

🔗 Related Error Codes

501 5.1.3
Invalid Address Syntax
500
SMTP Syntax Error
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