Email checker

<?php
/**
 * Plugin Name: Email Checker
 * Description: Checks the billing email address on order created and displays status in the admin order page.
 * Version: 1.0
 * Author: Your Name
 */

// Check Email Existence
function check_email_existence($email) {
    list($user, $domain) = explode('@', $email);
    $host = gethostbyname($domain);
    $port = 25;
    $timeout = 10;

    $socket = fsockopen($host, $port, $errno, $errstr, $timeout);
    if (!$socket) {
        return false;
    }

    $response = fread($socket, 1024);
    if (!preg_match('/^220/', $response)) return false;

    // EHLO request
    fwrite($socket, "EHLO $host\r\n");
    $response = fread($socket, 1024);
    if (!preg_match('/^250/', $response)) return false;

    // MAIL FROM request
    fwrite($socket, "MAIL FROM: <check@$host>\r\n");
    $response = fread($socket, 1024);
    if (!preg_match('/^250/', $response)) return false;

    // RCPT TO request
    fwrite($socket, "RCPT TO: <$email>\r\n");
    $response = fread($socket, 1024);
    if (!preg_match('/^250/', $response)) return false;

    // QUIT request
    fwrite($socket, "QUIT\r\n");
    fclose($socket);

    return true;
}

// Verify email on order creation and update post meta
add_action('woocommerce_new_order', 'verify_email_on_order_created');

function verify_email_on_order_created($order_id) {
    $order = wc_get_order($order_id);
    $email = $order->get_billing_email();
    $email_exists = check_email_existence($email);

    if (!$email_exists) {
        update_post_meta($order_id, '_billing_email_invalid', 'yes');
    } else {
        update_post_meta($order_id, '_billing_email_valid', 'yes');
    }
}

// Add meta box to display email verification status in the admin order edit page
add_action('add_meta_boxes', 'add_email_checker_meta_box');

function add_email_checker_meta_box() {
    add_meta_box('email-checker-meta-box', 'Email Verification', 'email_checker_meta_box_content', 'shop_order', 'side', 'high');
}

function email_checker_meta_box_content($post) {
    $email_valid = get_post_meta($post->ID, '_billing_email_valid', true);
    $email_invalid = get_post_meta($post->ID, '_billing_email_invalid', true);

    if ($email_valid) {
        echo "Email is valid.";
    } elseif ($email_invalid) {
        echo "Email is invalid.";
    } else {
        echo "Email hasn't been checked yet.";
    }
}