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

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

Sam Wheat 501

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

Bryan Trach 17,837 Reputation points Microsoft Employee Moderator

2024-07-06T02:21:26.66+00:00

@Sam Wheat To troubleshoot the "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" error, you can try the following steps:

Verify that the SMTP settings are correct, including the SMTP server address, port number, and SSL settings.

Confirm that the credentials used to authenticate the client are correct and have the necessary permissions to send email.

Check for any firewall restrictions that might be blocking the client's connection to the SMTP server. Ensure that the required ports are open for outbound traffic.

Ensure that the DNS settings are correct and that the client is able to resolve the domain name of the SMTP server correctly.

Check if the SMTP server requires authentication using a specific mechanism such as OAuth2. If so, ensure that the client is configured to use the correct authentication mechanism.

Regarding the different DNS domains, it should not cause an authentication error as long as the SMTP server is able to resolve the domain name correctly.

Let us know the outcome after you check the above 5 items so we can assist you further.

1 answer

Your answer

Bryan Trach 17,837 Reputation points Microsoft Employee Moderator

2024-07-06T02:21:26.66+00:00

@Sam Wheat To troubleshoot the "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" error, you can try the following steps:

Verify that the SMTP settings are correct, including the SMTP server address, port number, and SSL settings.

Confirm that the credentials used to authenticate the client are correct and have the necessary permissions to send email.

Check for any firewall restrictions that might be blocking the client's connection to the SMTP server. Ensure that the required ports are open for outbound traffic.

Ensure that the DNS settings are correct and that the client is able to resolve the domain name of the SMTP server correctly.

Check if the SMTP server requires authentication using a specific mechanism such as OAuth2. If so, ensure that the client is configured to use the correct authentication mechanism.

Regarding the different DNS domains, it should not cause an authentication error as long as the SMTP server is able to resolve the domain name correctly.

Let us know the outcome after you check the above 5 items so we can assist you further.

Answer 1

Sam Wheat 501

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

Thanks!

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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

1 answer

Your answer