SMTP 服务器需要安全连接或客户端未通过身份验证。(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("hajjaj@alhajjaji.com", "*******");

        // Set up the email message
        MailMessage mailMessage = new MailMessage();
        mailMessage.From = new MailAddress("hajjaj@alhajjaji.com");
        mailMessage.To.Add("hajjaj@gmail.com");
        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.

Answer 2

Sounds like the SMTP authentication of your Office 365 account hasn't been enabled. The third-party application uses SMTP submission to authenticate Office 365 account. To enable the SMTP auth, you can refer to the Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online article.