SMTP Server Authentication Error: How Can I Troubleshoot and Resolve '5.5.1 Authentication Required'?

SMTP Server Authentication Error: Troubleshooting and Resolution

This article will provide a comprehensive solution to the common SMTP server authentication error "5.5.1 Authentication Required".

This error message indicates that the SMTP server requires a secure connection or client authentication. This means that you need to establish a secure connection and provide valid credentials to authenticate to the SMTP server before you can send an email.

To resolve this error, please follow these steps:

1. Enable low-security apps in your Google Account:

   • Sign in to your Google Account and visit https://www.php.cn/link/0a8a64171a9145f6a47e5b3544632e3b or https://www.php.cn/link/96a83c96abbe6d0b40c72b279ebdf76e.
   • Enable the "Low-security apps" feature to allow access from low-security apps, including SMTP clients.

2. Configure SMTP credentials:

   • In your code, you need to specify the correct SMTP server, port, and credentials. Make sure you are using the correct username and password for your email account.
   • You can use the following code snippet:

<code class="language-csharp">using (MailMessage mail = new MailMessage())
{
    mail.From = new MailAddress("[email protected]");
    mail.To.Add("[email protected]");
    mail.Subject = "Hello World";
    mail.Body = "<h1>Hello</h1>";
    mail.IsBodyHtml = true;

    using (SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587))
    {
        smtp.Credentials = new NetworkCredential("[email protected]", "password");
        smtp.EnableSsl = true;
        smtp.Send(mail);
    }
}</code>

3. Verify connection settings:

   • Make sure your code uses the correct SMTP server settings. For Gmail, use "smtp.gmail.com" and port 587.
   • Make sure your code uses TLS encryption by setting EnableSsl to true.

4. Disable firewall or anti-virus software:

   • In some cases, firewalls or antivirus software may block outbound connections to SMTP servers. Temporarily disable these applications to test whether they are blocking email delivery.

5. Check Web.config settings:

   • In your Web.config file, verify that the SMTP credentials, server settings, and enableSsl values ​​are correct.

Follow these steps and you should be able to resolve SMTP server authentication errors and successfully send emails.

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