using System.Net.Mail;

            SmtpClient SmtpServer = new SmtpClient();
            SmtpServer.Credentials = new System.Net.NetworkCredential("User gmail", "Paaword");
            SmtpServer.Port = 587;
            SmtpServer.Host = "smtp.gmail.com";
            SmtpServer.EnableSsl = true;
            mail = new MailMessage();
            String[] addr = TextBox1.Text.Split(',');
            try
            {
                mail.From = new MailAddress("user gmail", "test", System.Text.Encoding.UTF8);
                Byte i;
                for( i = 0;i< addr.Length; i++)
                    mail.To.Add(addr[i]);
                mail.Subject = TextBox3.Text;
                mail.Body = TextBox4.Text;
                if(ListBox1.Items.Count != 0)
                {
                    for(i = 0;i<ListBox1.Items.Count;i++)
                    mail.Attachments.Add(new Attachment(ListBox1.Items[i].ToString()));
                }
                mail.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
                mail.ReplyTo = new MailAddress(TextBox1.Text);
                SmtpServer.Send(mail);
            }
            catch (Exception ex)
            {
            MessageBox.Show(ex.ToString());
            }