The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.3 Authentication unsuccessful

Question

I recently configured Communications Service and Email Communications Service.

I have two websites - I can send email from one but not the other. Both sites use the same binary to send email and the same connection string. The site that fails gives me this error:

System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.3 Authentication unsuccessful

Both sites are in the same subscription, resource group, and location. The sites have different dns domains - would that give me an authentication error?

How can I troubleshoot this? I have already seen these issues :

https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/fix-issues-with-printers-scanners-and-lob-applications-that-send-email-using-off#error-authentication-unsuccessful

https://learn.microsoft.com/en-us/answers/questions/1622311/azure-app-service-office-365-exception-the-smtp-se

https://learn.microsoft.com/en-us/answers/questions/681904/smtp-authenication-failure-5-7-57-and-5-7-139

Answer 1

You fixed it. I tried it again and it works. I've have changed nothing.

Thanks!