Enhanced SMTP status code · RFC 3463 · IANA
5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM
An application or device sends through smtp.office365.com without proper authentication; check its configuration.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
How 5.7.57 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.7.57
Client was not authenticated to send anonymous mail during MAIL FROM
An application or device sends through smtp.office365.com without proper authentication; check its configuration.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026