var search_str = /^[\w\-\.]+@[\w\-\.]+(\.\w+)+$/;

 var email_val = $("#Email").val();

 if(!search_str.test(email_val)){

     alert("please input right email !");

     $('#Email').focus();

     return false;

 }

ar mail = $('#mail').val();

 if (mail != '') {//判断

    var reg = /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+((\.[a-zA-Z0-9_-]{2,3}){1,2})$/;

    if (!reg.test(mail)) {

        alert('邮箱格式不正确，请重新填写!');

       return false;

  }

}

<script src="/js/jquery-1.4.2.js" type="text/javascript"></script>

<script language="javascript">$(document).ready(function() {

  $('.error').hide();

  $('.submit').click(function(event){

   var email=$('.email').val();

   if(valid_email(email)) {

    $('.error').hide();

   } else {

    $('.error').show();

    event.preventDefault();

   }

  });

 });