Erreur serveur sortant.

        private void Envoyer_Click(object sender, EventArgs e)
        {
            try
            {
                MailMessage mail = new MailMessage(from.Text,to.Text,subject.Text,Body.Text);
                SmtpClient client = new SmtpClient(smtp.Text);
                client.Port = 587;
                client.Credentials = new System.Net.NetworkCredential(username.Text,password.Text);
                client.EnableSsl = false;
                client.Send(mail);
                
                MessageBox.Show("message a été envoyée");
            }
            catch(Exception ex )
            {
                MessageBox.Show(ex.Message);
            }
        }