Q:

jQuery Ajax Post() geting responce with zero.. why?

I am working with word press . to submit a form with Ajax i use this code .. my php code is working fine and also j Query.
But when i am getting responce from post() and printing it.. 
it is printing with 'zero' means '0'..
As "Mail send 0"
PHP Code
 /* Apply Online form enqueue/localize/add action/function script start*/

    wp_register_script("ajax-apply_online", get_bloginfo('template_directory')."/js/custom_js.js", array( "jquery"));
    wp_enqueue_script('ajax-apply_online');
    wp_localize_script("ajax-apply_online","the_ajax_apply_online", array("ajaxurl_apply_online" => admin_url("admin-ajax.php")));

// add actions
   add_action( "wp_ajax_apply_online", "ajax_action_apply_online" );
   add_action( "wp_ajax_nopriv_apply_online", "ajax_action_apply_online" );

   function ajax_action_apply_online(){
     echo "Mail Send";                  
  }
/* Apply Online  form enqueue/localize/add action/function script end*/

jQuery Code
 jQuery.post(the_ajax_apply_online.ajaxurl_apply_online,
                    jQuery("#apply_online_form").serialize(),
                        function(response_from_the_action_function){

                            jQuery("#errfrmMsg").val(response_from_the_action_function).css({"display":"block"});
                        }
            );

A:

try adding exit; after echo "Mail Send"; 
If some other part of the php that executes after the mail send is adding additional text this will stop it from running

