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Bounce Decoder: 501 5.1.8

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501 5.1.8 — Bad Sender Address

The 501 5.1.8 error means your sending (From/Return-Path) address is invalid or rejected by the receiving server. Learn how to fix this.

🔴
Permanent Failure (Hard Bounce)
SMTP Code
501
Enhanced Code
5.1.8
Category
Bad Sender Address
Frequency
Very Common

🔢 Enhanced Status Code Breakdown: 5.1.8

Component Value Meaning
Class 5 Permanent failure
Subject 1 Addressing
Detail 8 Bad Sender Address

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

💬 What This Error Means

The receiving server rejected your message because your sender address (the From: or Return-Path: address) is invalid, malformed, or blocked. This is a problem with who you're sending from, not who you're sending to.

Common Causes

  • The Return-Path (envelope sender) address is malformed
  • Your sending domain does not exist in DNS
  • The receiving server has blocked your sender domain
  • Null sender address (<>) used incorrectly outside of bounce messages

How to Fix This

  • Verify your From: address is a valid, working email address
  • Ensure your sending domain has working MX and A records
  • Check that your email sending software is correctly configured with a valid sender
  • If using a third-party sender, verify your domain is properly set up in their dashboard

🏢 Provider-Specific Variations

Microsoft 365 / Outlook

Microsoft 365 returns this when the sender domain cannot be found in DNS. Ensure your domain has valid DNS records.

📖 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. 

501 5.1.8 Bad sender's system address
501 5.1.8 Domain of sender address does not exist

🔗 Related Error Codes

550 5.1.0
Other Address Status
550 5.7.1
Message Rejected by Policy

📚 Official Documentation

↗ RFC 3463 — Enhanced Status Codes
🔍

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