Microsoft 365 5.7.57
Client was not authenticated to send anonymous mail during MAIL FROM
Microsoft 365 5.7.57 means: client was not authenticated to send anonymous mail during MAIL FROM. 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
Client was not authenticated to send anonymous mail during MAIL FROM
Possible cause
You configured an application or device to send (relay) email messages in Microsoft 365 or Office 365 using the smtp.office365.com endpoint, and an issue affects the configuration of the application or device.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.57: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
550 5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM appears when a device or app sends through smtp.office365.com without logging in. Common causes are a scanner or line-of-business app configured without credentials, the wrong port, or a disabled SMTP AUTH setting on the mailbox. Microsoft is retiring basic authentication for SMTP AUTH, which will cause more of these for older devices. Check that the app uses port 587 with STARTTLS, a licensed mailbox, and that Authenticated SMTP is enabled for that mailbox in the admin center. If the device cannot authenticate, Microsoft documents direct send and SMTP relay through a connector as alternatives.
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 5.7.57?
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 5.7.57 mean?
- Microsoft 365 5.7.57 means: client was not authenticated to send anonymous mail during MAIL FROM. 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 5.7.57?
- 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 5.7.57?
- 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)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05