SMTP gmx server problam - smtp

Question

Im tyring to send an email with my program to an gmx email. Every time I try to send the mail I get the same error message in my console.
What can be the solution for that?
The error message:
System.Net.Mail.SmtpException: SMTP server requiers secure connection or the client wasnt authenticated. server response was: authentication required.
in - System.Net.Mail.SendMailAsyncResult.End(IAsyncResult result)
in - System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.Net.Mail;
using System.Net.Mime;
using System.Threading;
using System.ComponentModel;
namespace SMTP_Client
{
class Program
{
static bool mailSent = false;
private static void SendCompletedCallback(object sender, AsyncCompletedEventArgs e)
{
// Get the unique identifier for this asynchronous operation.
String token = (string)e.UserState;
if (e.Cancelled)
{
Console.WriteLine("[{0}] Send canceled.", token);
}
if (e.Error != null)
{
Console.WriteLine("[{0}] {1}", token, e.Error.ToString());
}
else
{
Console.WriteLine("Message sent.");
}
mailSent = true;
}
public static void Main(string[] args)
{
SmtpClient client = new SmtpClient("smtp.gmx.com",25);
MailAddress from = new MailAddress("example#project.com", "me " + (char)0xD8 + " you", System.Text.Encoding.UTF8);
MailAddress to = new MailAddress("example#gmx.com");
MailMessage message = new MailMessage(from, to);
message.Body = "The project has succeeded ";
message.Subject = "made it!";
client.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);
string userState = "test message2\n";
client.SendAsync(message, userState);
Console.WriteLine("Sending message... press c to cancel mail. Press any other key to exit.");
string answer = Console.ReadLine();
if (answer.StartsWith("c") && mailSent == false)
{
client.SendAsyncCancel();
}
message.Dispose();
Console.WriteLine("proccess ended.");
}
}
}