5.7.57: Client not authenticated to send anonymous mail
Exchange Online returns 5.7.57 when an app or device sends through smtp.office365.com without authenticating first. To fix it, configure the application to log in (SMTP AUTH, port 587, STARTTLS) with a licensed mailbox, or use a connector-based relay instead of anonymous submission.
The exact message
Microsoft 365 / Exchange Online
5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM
Why it happens
- The application has no username and password (or OAuth token) configured.
- The app connects without STARTTLS, so authentication never happens.
- SMTP AUTH is disabled for the mailbox or the tenant.
How to fix it
- Enable authentication in the app, using smtp.office365.com on port 587 with STARTTLS.
- Check that SMTP AUTH is enabled for the mailbox.
- For devices that cannot authenticate, set up a Microsoft 365 connector for relay.
If you send cold email
Cold email and warmup tools that connect Microsoft 365 mailboxes over SMTP need SMTP AUTH or OAuth working first. This is a setup error, not a reputation problem.
How WarmHawk handles it
WarmHawk sends through your own Google Workspace or Microsoft 365 mailboxes. When the provider stops accepting a mailbox’s sign-in (approval removed, password reset, no Exchange Online license), WarmHawk marks that mailbox as needing a reconnect and says why, instead of showing it as connected while every send fails. How WarmHawk works →
Related bounce codes
Sources, checked 2026-09-29: Microsoft Learn: NDRs and SMTP errors in Exchange Online · Microsoft Learn: Enable or disable SMTP AUTH in Exchange Online. Have a different bounce? Paste it into the decoder →
Questions
5.7.57 questions
What does 5.7.57 mean?+
Exchange Online returns 5.7.57 when an app or device sends through smtp.office365.com without authenticating first. To fix it, configure the application to log in (SMTP AUTH, port 587, STARTTLS) with a licensed mailbox, or use a connector-based relay instead of anonymous submission.
Is 5.7.57 a temporary or permanent error?+
Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.
How do I fix 5.7.57?+
Enable authentication in the app, using smtp.office365.com on port 587 with STARTTLS. Check that SMTP AUTH is enabled for the mailbox. For devices that cannot authenticate, set up a Microsoft 365 connector for relay.