Microsoft 365 SMTP submission error

Fix Outlook 550 5.7.57 client not authenticated

Microsoft documents 5.7.57 for an application or device sending anonymous mail during MAIL FROM through a Microsoft 365 configuration that requires authentication.

Problem description

The NDR reports:

550 5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM

Diagnostic availability: this is SMTP submission and tenant configuration. Do not paste credentials into MailFixIt; the public DNS checker is not applicable.

Is this temporary or permanent?

The response is permanent until the client uses the intended authenticated or connector-based route.

Probable causes

The application points to smtp.office365.com without supported authentication.
Credentials, mailbox permissions, or tenant SMTP AUTH policy changed.
A device intended for direct send or relay uses the submission endpoint.
Port, TLS, or authentication method is misconfigured.

Resolution sequence

Identify the approved Microsoft 365 send method for the device.
Compare endpoint, port, TLS, identity, and authentication requirements.
Confirm tenant and mailbox policies without exposing secrets.
Use a scoped connector only when organizational design requires relay.
Send a minimal test and inspect Microsoft message trace.

Frequently asked questions

What does Microsoft 5.7.57 mean?

An application or device attempted to send anonymous mail during MAIL FROM through a Microsoft 365 submission configuration that requires authentication.

Will an SPF change fix it?

No. This is SMTP client submission or relay configuration, not public SPF authorization.

Should I put credentials in a public checker?

Never. Configure secrets only in the trusted sending application or device and use the organization's approved authentication method.

Related email diagnostics

Primary source: Microsoft Exchange Online NDR reference.