Visual Basic.NET - Error al enviar correo

Private Sub BEnvoi_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BEnvoi.Click

        Dim Mail As MailMessage

        Dim smtp As SmtpClient

        Dim Expediteur As String

        Dim Destinataire As String

        Try

            Mail = New MailMessage

            Mail.SubjectEncoding = UTF8Encoding.UTF8 ' Forma de codigo del objecto

            Mail.BodyEncoding = UTF8Encoding.UTF8 ' Forma de codigo del mensaje

            Mail.Subject = TObjet.Text  ' el objecto del mensaje en un TextBox

            Mail.Body = RTBMessage.Text ' el mensaje en un RichTextBox

            smtp = New SmtpClient("smtp.gmail.com") ' servidor SMTP permitido en el ordinador

            Expediteur = "scxster@gmail.com"

            Mail.From = New MailAddress(Expediteur) 'el remitente

            Destinataire = "scx@belgacom.net"

            Mail.To.Add(New MailAddress(Destinataire)) ' el destinatario

            smtp.Port = "587"

            smtp.Credentials = New System.Net.NetworkCredential("scxster", "Password")  'scxster es el nombre del usuario

            smtp.EnableSsl = True

            For Each S As String In LBAttachement.Items  ' Nombres de los archivos a adjuntar

                Dim Attache As New Attachment(S)

                Mail.Attachments.Add(Attache)

            Next

            smtp.Send(Mail) ' Enviar mensaje

            Mail.Dispose()

        Catch ex As ApplicationException

            MessageBox.Show(ex.Message)

        Finally

            smtp = Nothing

            Try

                Mail.Dispose()

            Catch

            End Try

            Mail = Nothing

        End Try

    End Sub