Microsoft 365 5.1.0
Sender denied
Microsoft 365 5.1.0 means: sender denied. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Sender denied
Possible cause
This NDR commonly occurs when someone saves an email message to a file in Outlook, opens it offline, and then replies. The message property preserves only the legacyExchangeDN attribute when Outlook delivers the message. Therefore, the lookup could fail.
Provider's advice
Either the recipient address is incorrectly formatted or the recipient couldn't be correctly resolved. The first step to resolve this error is to check the recipient address, and send the message again.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.1.0: Other address status |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 5.1.0?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 5.1.0 mean?
- Microsoft 365 5.1.0 means: sender denied. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 5.1.0?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Would verifying my list have prevented Microsoft 365 5.1.0?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05