error :The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required

Can someone help please. 
Imports System.Data Imports System.Data.SqlClient Imports System.Configuration Imports System.Net.Mail 'Imports System.Net.Mail.SmtpClient Public Class customer_regis Inherits System.Web.UI.Page Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("recycling").ConnectionString) Protected Sub btn_cus_submit_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btn_cus_submit.Click Try Const dummy As String = "test@test.test" Dim message As String message = "Dear " + txt_f_name.Text + "," + vbCrLf + vbCrLf message += "Thank you for registering. Before we can activate your account one last step must be taken to complete your registration." + vbCrLf + vbCrLf message += "To complete your registration, please visit this U ...