ASP.net wizard Form using smtp

I wanted to create a feed back form using asp.net wizard function. I tried watching video and made the page but at the end when I tried to send the message I get an error.

The SMTP sever requires a secure connection or the client was not authenticated.
The server response was 5.7.0

I did expect this error becuase when you look at the code below, I did not find a place to put a password or a port number.

NEED HELP.

Imports System.Net.Mail
 
 
Partial Class _Default
    Inherits System.Web.UI.Page
 
    Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles CustomValidator1.ServerValidate
        If txtComments.Text.Length > 10 Then
            args.IsValid = False
        Else
            args.IsValid = True
        End If
    End Sub
 
    Protected Sub Wizard1_FinishButtonClick(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.WizardNavigationEventArgs) Handles Wizard1.FinishButtonClick
        SendMail(txtEmail.Text, txtComments.Text)
    End Sub
 
    Private Sub SendMail(ByVal from As String, ByVal body As String)
        Dim mailServerName As String = "smtp.gmail.com"
        Dim message As MailMessage = New MailMessage(from, "myemail@gmail.com", "feedback", body)
        Dim mailClient As SmtpClient = New SmtpClient
 
        mailClient.Host = mailServerName
        mailClient.Send(message)
        message.Dispose()
    End Sub
End Class

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