could not connect smtp server

 hi,
i am learner and watching videos and trying to do something, i got two question :
First question.

"create contact us page" video and  i did everything but i got error after pressing finish button.
error says : The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first. 24sm668890eyx.48
my code is below:
Imports System.Net.Mail
Partial Public Class WebForm1
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    End Sub

    Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles CustomValidator1.ServerValidate
        If TextC ...