C#: The SMTP Server Requires A Secure Connection Or The Client Was Not Authenticated. The Server Response Was: 5.5.1 Authentication Required?

This error message indicates that the SMTP server you are trying to connect to requires authentication before sending emails. To resolve this issue, you need to provide authentication credentials (i.e., username and password) to the SMTP server.

Here’s an example of how you can authenticate with an SMTP server using C#:

using System;
using System.Net;
using System.Net.Mail;

class Program
    static void Main()
        // SMTP server details
        string smtpServer = "smtp.example.com";
        int smtpPort = 587;
        string smtpUsername = "your_username";
        string smtpPassword = "your_password";

        // Sender and recipient email addresses
        string senderEmail = "[email protected]";
        string recipientEmail = "[email protected]";

        // Create the SMTP client and set authentication details
        SmtpClient smtpClient = new SmtpClient(smtpServer, smtpPort);
        smtpClient.Credentials = new NetworkCredential(smtpUsername, smtpPassword);
        smtpClient.EnableSsl = true; // Enable SSL encryption for secure connection

            // Create a new email message
            MailMessage mail = new MailMessage(senderEmail, recipientEmail, "Subject", "Body");

            // Send the email

            Console.WriteLine("Email sent successfully.");
        catch (Exception ex)
            Console.WriteLine("Failed to send email: " + ex.Message);

Make sure to replace the placeholder values (smtp.example.com, your_username, your_password, [email protected], and [email protected]) with your actual SMTP server details and email addresses.

Note: If you encounter a "530 5.7.0 Must issue a STARTTLS command first" error, it means that the server requires a secure connection. You can resolve this by setting smtpClient.EnableSsl = true; as shown in the example above.

About the Author Rex

I'm a passionate tech blogger with an insatiable love for programming! From my early days tinkering with code, I've delved into web dev, mobile apps, and AI. Sharing insights and tutorials with the world is my joy, connecting me to a global community of like-minded tech enthusiasts. Python holds a special place in my heart, but I embrace all challenges. Constantly learning, I attend tech conferences, contribute to open-source projects, and engage in code review sessions. My ultimate goal is to inspire the next generation of developers and contribute positively to the ever-evolving tech landscape. Let's code together!