textbox text1;
textbox text2;

public void invio()
        {
            if (text1.Text.Length == 0 && text2.Text.Length == 0)
            {
                MessageBox.Show("Please Fill The \"From\" and The \"To\" Field.", "Missing", MessageBoxButton.OK, MessageBoxImage.Exclamation);
                return;  
            }
            if (text1.Text.Length == 0)
            {
                MessageBox.Show("Please Fill The \"From\" Field.", "Missing", MessageBoxButton.OK, MessageBoxImage.Exclamation);
                return;
            }
            if (text2.Text.Length == 0)
            {
                MessageBox.Show("Please Fill The \"To\" Field.", "Missing", MessageBoxButton.OK, MessageBoxImage.Exclamation);
                return;
            }
            progressBar1.Value = 0;
            progressBar1.Minimum = 0;
            progressBar1.Maximum = Convert.ToInt32(comboBox2.SelectedItem.ToString());
            int nom = Convert.ToInt32(comboBox2.SelectedItem.ToString());
          
            for (int i = 0; i <= nom; i++)
            {
                try
                {
                    this.Title = "UFO Mail - Sending...";
                    Cursor = Cursors.Wait;
                    MailMessage Msg = new MailMessage();
                    Msg.From = new MailAddress(textBox1.Text.ToString());
                    Msg.to.Add(new MailAddress(textBox2.Text.ToString()));
                    Msg.Subject = textBox3.Text.ToString();
                    Msg.Body = richTextBox1.Document.ToString();
                    Msg.IsBodyHtml = true;
                    SmtpClient Smtp = new SmtpClient(comboBox1.SelectedItem.ToString());
                    Smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
                    NetworkCredential Credential = new NetworkCredential();
                    Smtp.UseDefaultCredentials = false;
                    Smtp.Credentials = Credential;
                    Smtp.Send(Msg);
                    progressBar1.Value = +1;
                    if (progressBar1.Value == nom)
                    {
                        MessageBox.Show("message Sent!", "Sent", MessageBoxButton.OK, MessageBoxImage.Information);
                        this.Title = "UFO Mail";
                        Cursor = Cursors.Arrow;
                        progressBar1.Value = 0;
                    }
                }
                catch
                {
                    MessageBox.Show("Houston, We Have a Problem!", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                    this.Title = "UFO Mail";
                    Cursor = Cursors.Arrow;
                    progressBar1.Value = 0;
                    return;
                }
            }
        }
        private void button1_Click(object sender, RoutedEventArgs e)
        {

 // prima di avviare il thread ti salvi i riferimenti
text2 = textbox2
text1 = textbox1 
            Thread myThread = new Thread(new ThreadStart(invio));
            myThread.Start();
        }