MailSender ms = new MailSender("smtp.list.ru", 25, "BiLL@list.ru", "password");

Thread mainThread = new Thread(delegate()
{
         this.button1.Enabled = false;                
                
                if (ms.send("XXX@list.com", "This is a test", textBox1.Text)) 
                   MessageBox.Show("Отправилось!");
                else 
                   MessageBox.Show(ms.getErrMsg(), "Информация");

         this.button1.Enabled = true;
}) 

mainThread.Name = "Sending email";
mainThread.Start();            

System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;

Thread mainThread = new Thread(delegate()
{
          closeToolStripButton.Enabled = false;  //вот сдесь выдает ошибку!
          SendMessageForProcess(emails, message, ref mailSend, ref error);
          closeToolStripButton.Enabled = true;
});

mainThread.Name = "Sending email";
mainThread.Start();