SMTP Error 5.5.1: Authentication Required: How Can I Resolve This Email Sending Issue?

SMTP Error 5.5.1: Authentication required

Question:

When your application sends an email, you encounter the error "The SMTP server requires a secure connection or the client is not authenticated. The server responded with: 5.5.1 Authentication required". What is the cause of this error and how to fix it?

Answer:

This error usually means that the given SMTP server requires authentication. Here are some possible solutions:

1. Enable less secure app access:

   Google may have blocked sign-in attempts from less secure apps or devices. To resolve this issue, enable "Less secure app access" (https://www.php.cn/link/96a83c96abbe6d0b40c72b279ebdf76e) in your Google Account settings.

2. Use SSL:

   Make sure to use SSL to establish a secure connection to the SMTP server. When configuring SmtpClient, set the EnableSsl property to true.

3. Provide credentials:

   Make sure the NetworkCredential object is set correctly with a valid username and password. Confirm that the specified username matches the email address used in the MailMessage's From field.

4. Use SMTP port:

   Check that the specified SMTP port (usually 587 or 465) is correct and matches the settings specified in the SMTP server configuration.

5. Configuration Web.config:

   If you have configured SMTP settings in Web.config, verify that the node contains the correct values ​​for host, port, username, and password.

6. Disable UseDefaultCredentials:

   Try setting UseDefaultCredentials to false and specifying the credentials explicitly in the SmtpClient.

7. Check third-party antivirus or firewall software:

   These tools may interfere with SMTP connections. Temporarily disable or configure them to allow access to the SMTP server.

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