Microsoft mail servers 550 5.7.57
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
Do not retry unchanged. Fix the sender setup first.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- MAIL FROM
- Endpoint
- SMTP submission
Client was not authenticatedMAIL FROMUse 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
- Treat this as a sender setup incident, not a bad recipient.
- Hold similar mail for the same sender domain or provider until the fix is verified.
- Retry after a fresh single-recipient test succeeds.
Avoid
- Do not retry unchanged in a loop.
- Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
- Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
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
- smtp client not authenticated
- Evidence
- official