Hi @Dinesh ,

Use below script include :-

var EmailValidation = Class.create();
EmailValidation.prototype = Object.extendsObject(AbstractAjaxProcessor, {

checkExistingUser: function() {
var emailaddress = this.getParameter('sysparm_email');
var gr = new GlideRecord("sys_user"); //made mistake in user table name. now corrected.
gr.addQuery("email", emailaddress);
gr.query();
if (gr.next()) {
return  true;
}else{
return  false;
}
},
type: 'EmailValidation'
});