How to resolve this issue. I am getting error while sending emails

Question

Error: 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 5.7.139 Authentication unsuccessful, basic authentication is disabled. [PN3PR01CA0011.INDPRD01.PROD.OUTLOOK.COM 2024-09-23T08:56:54.313Z 08DCDB83814A4335]

Getting the above error from today onwards while sending mails using this server "smtp.office365.com"

Here is the detailed error getting in C# 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. [PN2PR01CA0089.INDPRD01.PROD.OUTLOOK.COM 2024-09-23T11:45:32.835Z 08DCDB31B8645AFE]'

Answer 1

I have same issue. I'm using email personal account I cannot access admin portal.

Answer 2

Hi,@Shiva Maurya. Welcome to Microsot Q&A.

There may be other two possible causes of this issue. Please refer to this link and check point 3 (Azure Security Defaults) and point 4 (Conditional Access policy): Error: Authentication unsuccessful

These settings would block Legacy Authentication.

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