window.onload = checkMail;  
function checkMail() {
  document.getElementById('submit-newsletter').onclick = function() {       
    var exp = new RegExp('^[a-zA-Z0-9]+[.a-zA-Z0-9_-]*@[a-zA-Z0-9_-]+[.a-zA-Z0-9_-]*\\.[a-zA-Z0-9_-]{2,4}$');    
    var mail = document.getElementById('email-subscriber').value;    
    if(!mail.match(exp)) {
      alert("Your email is incorrect!");
      return false;   
    }
  };
}