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Bounce Decoder: 501

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

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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.

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

💬 What This Error Means

A parameter in the SMTP command was invalid — usually a malformed sender or recipient address in the SMTP envelope (separate from the visible From/To headers).

Common Causes

  • Invalid characters in the SMTP MAIL FROM or RCPT TO address
  • Non-standard address format in the envelope

How to Fix This

  • Check the sender address format
  • Ensure envelope addresses are valid RFC 5321 format

📋 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

📚 Official Documentation

↗ RFC 5321 — SMTP
🔍

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