How to check whether email address already exists in database using ASP.Net AJAX

Hi...

 I want to validate the email id in cliens side, so that to prevent the postback of page to server,

and every user shoud registerd with email id... so i want to validate email id at client side.. i.e. shoud be check it is exist in the database or not

Regards 

shivanand (shivu.betta@gmail.com)