Custom Worpdress Plugin Call Ajax Return 0

Custom Worpdress Plugin Ajax Call return 0. Main File as Bellow.

<?php
/*
Plugin Name: XYZ
*/

class GMS_SMS_Notifier {


public function __construct(){
    if (is_admin()){
        //JS
        add_action( 'admin_enqueue_scripts',array($this,'load_custom_js_scripts'));
    }


function load_custom_js_scripts() {
    wp_register_script( 'gms_custom', plugins_url('/assets/js/gms_custom.js', __FILE__ ),false,'1.0',true);
    wp_localize_script('gms_custom','plugin_ajax',array('ajaxurl'=>admin_url('admin-ajax.php')));
    wp_enqueue_script('gms_custom');

}

function mail_send_to(){
    global $wpdb;
    echo $_POST['recordID'].'Hello';
    die();

add_action('wp_ajax_mail_send_to','mail_send_to');
add_action('wp_ajax_nopriv_mail_send_to','mail_send_to');

} // End Class GMS_SMS_Notifier

new GMS_SMS_Notifier();

Custom Function do not want to add function in theme function.php

JS file gms_custom.js as bellow

$(".send_mail_btn").click(function(){
var recordids = $(this).attr("id");

    $.ajax({
        url: plugin_ajax.ajaxurl,
        type: 'POST',
        data: ({
        action: 'mail_send_to',
        recordID: recordids,
        }),
        success: function(data) {
            console.log(data);
       }
   });
});

Problem: Custom Function put on theme function.php working fine.but when put on plugin file that not working. I wan't same file

Given me your suggestion.

Thanks.

via Chebli Mohamed