Microsoft mail servers 451 4.7.0
SMTP authentication
SMTP authentication means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.
Decision
A retry can work, especially after a short wait.
Confidence: medium. Recipient validity: unknown.
Match this reply
- SMTP stage
- Authentication
- Endpoint
- SMTP submission
Temporary server errorPRXauthenticationUse tools
Start here
- 1
Check SMTP credentials, OAuth token state, and whether SMTP AUTH is enabled for the mailbox or tenant.
- 2
Refresh credentials or reconnect the sending account before retrying.
Confirm the cause
- Confirm the SMTP credentials, OAuth scopes, and token expiry for the sending account.
- Check whether SMTP AUTH is enabled for the mailbox or tenant.
- Reconnect the account or refresh credentials, then test with one message before resuming volume.
App handling
- Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
- Slow or pause provider-specific sending if the same deferral repeats across many recipients.
- Escalate to reputation, authentication, or volume review when retries keep failing.
Avoid
- Do not suppress the recipient from a temporary response.
- Do not keep the same retry pace if the deferral repeats across many recipients.
- Do not ignore sender reputation or authentication if this keeps happening.
Common causes
- SMTP credentials, OAuth permissions, or access tokens are invalid or expired.
- SMTP AUTH is disabled for the mailbox, tenant, or account type.
- The SMTP client is authenticating against the wrong endpoint or account.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
- Specific cause
- temporary auth or proxy failure
- Evidence
- official