Error code 5.7.57 in a non-delivery report (also known as an NDR, bounce message, delivery status notification, or DSN). This bounce message indicates a problem in the configuration of the connecting application or device.
Some things you could check:
Triple-check the email/password.
Change password - it could be that the password is "corrupted" or "prematurely expired".
Turn on SMTP for the sending email account.
Multi-Factor Authentication is turned "OFF" for the sending email account.
Things to try:
Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes.
A DMARC reporting service is very helpful for identifying email sources and SPF failures for the domain. Ensure you are properly setting up SPF and DMARC.
You may try to enable network tracing. Review Network Tracing in the .NET Framework to see if it gives any additional information.
Troubleshooting & workarounds shared by the community members and external support:
5.7.57 SMTP - Client was not authenticated - some answers are dated but could still be applicable.