SoloCodigo

public static class Program
{
        public static void Main()
        {
            SendMailForm form = new SendMailForm();
            new Thread(ChangeText).Start(form);
            Application.Run(form);
        }

        private static void ChangeText(object formObj)
        {
            SendMailForm form = (SendMailForm) formObj;
         
            Delegate del = GetDelegate();

            form.Invoke(del, form, "texto a setear sobre el campo txtBody del form SendMailForm");
        }

        private static Delegate GetDelegate()
        {
               MethodInfo method = typeof(Program).GetMethod("ChangeTextBox");
               return Delegate.CreateDelegate(typeof (ChangeTextDel), method);
        }
}

       public delegate void ChangeTextDel(SendMailForm f, string text);
        public static void ChangeTextBox(SendMailForm f, string text)
        {
            f.txtBody.Text = text;
        }