Sending emails from a web form

Hello,

I am trying to make my asp.net web form send an email from @gmail.com to @gmail.com email address. Using code provided below. Obviously, "username@gmail.com" is replaced with the email address that sends an email and etc.

   Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Try
            Dim SmtpServer As New SmtpClient()
            Dim mail As New MailMessage()
            SmtpServer.Credentials = New  _
        Net.NetworkCredential("username@gmail.com", "password")
            SmtpServer.Port = 587
            SmtpServer.Host = "smtp.gmail.com"
            mail = New MailMessage()
            mail.From = New MailAddress("yourusername@gmail.com")
            mail.To.Add("recieveraddress")
            mail.Subject = "Test Mail"
            mail.Body = "This is for testing SMTP mail from GMAIL"
            SmtpServer.Send(mail)
            MsgBox("mail send")
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

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When the button is pressed, I receive the error message:


How can I fix it? Thanks!