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.
Published · Last verified · Maintained by TamingDNS
501
5.1.8
Bad Sender Address
Very Common
🔢 Enhanced Status Code Breakdown: 5.1.8
| Component | Value | Meaning |
|---|---|---|
| Class | 5 | Permanent failure (Hard bounce) |
| Subject | 1 | Address / Destination |
| 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
This is a 'From' problem, not a 'To' problem. The receiving server rejected your message because it didn't like your own email address. It might think your address is formatted incorrectly, or it can't find your domain in the global phonebook (DNS).
Common Causes
- There is a typo in your own "From" or "Return-Path" email address
- Your domain is missing important DNS records, so other servers can't verify who you are
- The receiving server has blocked your entire domain for some reason
- Your email software is sending "blank" sender information that the other server doesn't allow
How to Fix This
- Check your email settings to make sure your "From" address is spelled correctly
- Make sure your domain has working MX and A records in your DNS settings
- If you're using a newsletter service, make sure you've "authenticated" your domain in their dashboard
- Ask your IT team to verify that your mail server is identifying itself correctly
📚 Official Documentation
Microsoft 365 returns this when the sender domain cannot be found in DNS. Ensure your domain has valid DNS records.
📖 Microsoft 365 / Outlook documentation → Browse all Microsoft 365 / Outlook error codes →📋 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
🔧 Related Diagnostic Tools
These tools can help you diagnose and fix this type of bounce:
Got a bounce message to decode?
Paste your full NDR email, SMTP error line, or mail log fragment to get an instant plain-English diagnosis.
Open the Bounce Decoder →