如果使用jquery，Ajax信息无效，则将数据提交到数据库中

I am new to Jquery and ajax.I want to check if mail id is already exist in database. So I use Ajax to check email is exist or not. I check this condition during submitting a form, so I use onSubmit event. when I enter submit button the above condition is checked and then inserted into database.but If condition fails that email also inserted into database. what is the error? I cannot find this error please help me! my html code is:I am new to Jquery and ajax.I want to check if