function checkMail (form) {
       var emailAdres = form.email.value;
valid = new RegExp("^[a-zA-Z0-9]+([.]?[a-zA-Z0-9_-]+)*[@]([a-zA-Z0-9_-]+[.])+([a-z]{2,4})$").exec(emailAdres);
if(!valid) { 
   alert( "Vul een geldig email adres in" );
   form.email.focus();
   return false;
}
return true;
}