Microsoft 365 5.7.1
Delivery not authorized / Unable to relay / Client was not authenticated
Microsoft 365 5.7.1 means: delivery not authorized / Unable to relay / Client was not authenticated. 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
Delivery not authorized
Possible cause
The sender of the message isn't allowed to send messages to the recipient.
Provider's advice
This error occurs if the sender tries to send a message to a recipient but the sender isn't authorized to send one. This error frequently occurs when a sender tries to send messages to a distribution group that's configured to accept messages only from members of that distribution group or other authorized senders. The sender must request permission to send messages to the recipient. This error can also occur if an Exchange transport rule rejects a message because the message matched conditions that are configured on the transport rule.
Provider's description
Unable to relay
Possible cause
The sending email system isn't allowed to send a message to an email system that isn't the final destination of the message.
Provider's advice
This error occurs if the sending email system tries to send an anonymous message to a receiving email system, and the receiving email system doesn't accept messages for the domain or domains that are specified in one or more of the recipients. The following are the most common cause of this error: A third party tries to use a receiving email system to send spam, and the receiving email system rejects the attempt. By the nature of spam, the sender's email address might be forged, and the resulting NDR could have been sent to the unsuspecting sender's email address. It's difficult to avoid this situation. An MX record for a domain points to a receiving email system where that domain isn't accepted. The administrator responsible for the specific domain name must correct the MX record or configure the receiving email system to accept messages sent to that domain, or do both. A sending email system or client that should use the receiving email system to relay messages doesn't have the correct permissions to do this.
Provider's description
Client was not authenticated
Possible cause
The sending email system didn't authenticate with the receiving email system. The receiving email system requires authentication before message submission.
Provider's advice
This error occurs if the receiving server must be authenticated before message submission, and the sending email system isn't authenticated with the receiving email system. The sending email system administrator must configure the sending email system to authenticate with the receiving email system for delivery to be successful.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.1: Delivery not authorized, message refused |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
Microsoft documents three different 5.7.1 replies. Delivery not authorized means the recipient, often a distribution group, accepts mail only from approved senders, or a transport rule rejected the message. Unable to relay means the receiving system does not accept mail for that domain, typically because of a wrong MX record or relay permissions. Client was not authenticated means the receiving system requires authentication before accepting mail.
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 5.7.1?
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 5.7.1 mean?
- Microsoft 365 5.7.1 means: delivery not authorized / Unable to relay / Client was not authenticated. 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 5.7.1?
- 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 5.7.1?
- 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.
The same code elsewhere
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