Send email to specific user by using webform
Finally i found the solution by myself 

1) Create a hidden field in webform say username field.

2)then go to /webform/emails .select the component value that is equal to username hidden field you previously created.then edit added component value in E-mail from address section select the component radio button instead of Default: abc@yahoo.com radio button.

3)then create a custom module add this code 

 function your_module_form_alter(&amp;$form, &amp;$form_state, $form_id){

 if ($form_id == 'your_webform_id') {
    $uid = arg(1);
   if ( arg(0) == 'user' &amp;&amp; is_numeric($uid) ) {
       $account = user_load($uid);
        $form['submitted']['Your-hidden-field-machine-name']['#value']   = $account-&gt;mail;
     }
   }

   }