The SMTP server requires a secure connection or the client was not authenticated.(535 5.7.3)?

Question

I am totally fed-up with Office 365 SMTP configuration, I have done all requirements to send email using my account on Office 365, and always giving me the same error message :

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 [DX0P273CA0007.AREP273.PROD.OUTLOOK.COM 2023-06-01T20:34:23.527Z 08DB62D998FCDD7E]

• I have used direct password and did not work
• Used App password and activating the MFA and did not work.

the below code is used for sending the email which I don't think it has any issue

 SmtpClient smtpClient = new SmtpClient("smtp.office365.com", 587);
        smtpClient.EnableSsl = true;
        smtpClient.UseDefaultCredentials = false;
        smtpClient.Credentials = new NetworkCredential("[email protected]", "*******");

        // Set up the email message
        MailMessage mailMessage = new MailMessage();
        mailMessage.From = new MailAddress("[email protected]");
        mailMessage.To.Add("[email protected]");
        mailMessage.Subject = EmailSubject;
        mailMessage.Body = EmailMsg;
        mailMessage.IsBodyHtml = true;
        smtpClient.Send(mailMessage);

Application password is added as you can see

SMTP is enabled for that Mailbox also

SMTP Disabled status : not disabled any help will be very apricated. thanks

Answer 1

For smtp.office365.com to work, you need to do the following:

1. Login into the Microsoft 365 Admin Center as an admin user.
2. Click Azure Active Directory in the menu. If the option is not visible, click Show All. See image #1.
3. In the Azure Active Directory, click Properties on the page. See Image #2
4. At the bottom of the page, click Manage security defaults.
5. Turn it off and answer why you are turing it off, then click Save. # See image #3
6. Test your email code.

Image #1

Image #2

BTW, make sure you are using .NET Framework 4.8 or later and user's office 365 MFA option is disabled.