SmtpClient sending without authentication

Question

I am sending emails to our clients from Java. And there is no any authentication for our SMTP. So I use the following code in Java to send it without authentication:

Properties props = new Properties();
Session session;
props.put("mail.smtp.auth", "false");
session = Session.getInstance(props, null);

This code works fine for sending emails from Java. But I want to send emails using ASP.NET and C#. But I am unable to send it. For sending it using C# I am using the following code:

SmtpClient smtp = new SmtpClient();
smtp.Host = "<My smtp.Host>";
smtp.EnableSsl = false;
smtp.Credentials = CredentialCache.DefaultNetworkCredentials;
smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
smtp.Send(message);

But it gives me the following error:

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Relaying not allowed: <Here email address of To>

How to send it without authentication?

Answer 1

From Msdn. If the UseDefaultCredentials property is set to false and the Credentials property has not been set, then mail is sent to the server anonymously.