5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM
Sample bounce line
530 5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM [AM0PR0102CA0070.eurprd01.prod.exchangelabs.com]
What it means
An application or device was configured to relay through smtp.office365.com but did not authenticate, so Microsoft refused the MAIL FROM command. Microsoft's explanation is that the configuration of the application or device is at fault, not the mailbox.
Why it happens
- SMTP AUTH disabled on the mailbox the device signs in as, which Microsoft now disables by default for new tenants.
- No credentials configured on the device at all, so it connects anonymously.
- Basic authentication used where the tenant only allows modern authentication.
- A password changed on the mailbox and never updated on the device, which fails authentication and then falls back to anonymous.
How to fix it
- Enable SMTP AUTH for the specific mailbox the device uses, rather than for the whole tenant.
- Give the device a real sign-in: username, password and STARTTLS on port 587.
- Where the device cannot do modern authentication, use a dedicated mailbox for it and keep its credentials current.
- If the device only needs to send to recipients inside your own tenant, direct send or a connector may be the right route instead of authenticated submission.
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