[Help] Sync Gmail using smtp in asp.net using c#

public void SendMail()
        {
            //Build The MSG
            System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();
            
            msg.To.Add(textBox2.Text);
            msg.From = new MailAddress(textBox1.Text);
            msg.Subject = textBox3.Text;
            msg.SubjectEncoding = System.Text.Encoding.UTF8;
            msg.Body = richTextBox1.Text;
            msg.BodyEncoding = System.Text.Encoding.UTF8;
            msg.IsBodyHtml = false;
            msg.Priority = MailPriority.High;
            System.Net.Mail.Attachment attachment;
            attachment = new System.Net.Mail.Attachment(textBox4.Text);
            msg.Attachments.Add(attachment);

            //Add the Creddentials
            SmtpClient client = new SmtpClient();
            client.Credentials = new System.Net.NetworkCredential
                (textBox1.Text, "Password");
            client.Port = 587;//or use 587            
            client.Host = "smtp.gmail.com";
            client.EnableSsl = true;
            client.SendCompleted += new SendCompletedEventHandler
                (client_SendCompleted);
            object userState=msg;
            try
            {
                //you can also call client.Send(msg)
                client.SendAsync(msg, userState);                
            }
            catch (System.Net.Mail.SmtpException ex)
            {
                MessageBox.Show(ex.Message, "Send Mail Error");
            }
        }