function check_email() {
  var submitted = false;
  var error = 0;
  var error_message = "The contact form was not filled out completely or wrongly!\nPlease correct your details.\n\n";

  if(submitted){ 
    alert( "This form has already been submitted. Please press Ok and wait for this process to be completed."); 
    return false; 
  }
   
  var email = document.kontakt.txtEMail.value;  
  var name = document.kontakt.txtName.value;
  var firma = document.kontakt.txtFirma.value;
  var betreff = document.kontakt.txtBetreff.value;
  var filter=/^.+@.+..{2,3}$/;

  if (document.kontakt.elements['txtEMail'].type != "hidden") {
    if (email == '' || email.length < 8 || filter.test(email) == false) {
      error_message = error_message + "* It act himself around no valid e-mail address!.\n";
      error = 1;
    }
  }


  if (document.kontakt.elements['txtName'].type != "hidden") {
    if (name == '' || name.length < 2) {
      error_message = error_message + "* The name must at least consist of 2 letters.\n";
      error = 1;
    }
  }

  if (document.kontakt.elements['txtFirma'].type != "hidden") {
    if (firma == '' || firma.length < 2) {
      error_message = error_message + "* The company name must at least consist of 2 letters.\n";
      error = 1;
    }
  }

  if (document.kontakt.elements['txtBetreff'].type != "hidden") {
    if (betreff == '' || betreff.length < 4) {
      error_message = error_message + "* The subject must at least consist of 4 letters.\n";
      error = 1;
    }
  }


  if (error == 1) { 
    alert(error_message); 
    return false; 
  } else { 
    submitted = true; 
    return true; 
  } 
}