WordPress + MailOdds

// functions.php - Manual integration (if not using the plugin)
// Validates email on registration with caching and fail-open timeout

add_action('registration_errors', function($errors, $sanitized_user_login, $user_email) {
    // Skip if API key not defined
    if (!defined('MAILODDS_API_KEY')) {
        return $errors;
    }

    // Check transient cache (24h TTL)
    $cache_key = 'mailodds_' . substr(hash('sha256', strtolower($user_email)), 0, 16);
    $cached = get_transient($cache_key);

    if (false === $cached) {
        $response = wp_remote_post('https://api.mailodds.com/v1/validate', [
            'headers' => [
                'Authorization' => 'Bearer ' . MAILODDS_API_KEY,
                'Content-Type'  => 'application/json',
            ],
            'body'    => json_encode(['email' => $user_email]),
            'timeout' => 5,
        ]);

        // Fail-open: if API unreachable, allow registration
        if (is_wp_error($response)) {
            return $errors;
        }

        $cached = json_decode(wp_remote_retrieve_body($response), true);

        // Cache for 24 hours
        set_transient($cache_key, $cached, DAY_IN_SECONDS);
    }

    $action = isset($cached['action']) ? $cached['action'] : '';

    if ('reject' === $action) {
        $errors->add('invalid_email',
            '<strong>Error:</strong> This email address could not be verified.');
    }

    return $errors;
}, 10, 3);

// WooCommerce checkout validation (included in the plugin)
// Also validates on WooCommerce My Account registration

add_action('woocommerce_after_checkout_validation', function($data, $errors) {
    $email = isset($data['billing_email']) ? $data['billing_email'] : '';
    if (empty($email) || !defined('MAILODDS_API_KEY')) {
        return;
    }

    $response = wp_remote_post('https://api.mailodds.com/v1/validate', [
        'headers' => [
            'Authorization' => 'Bearer ' . MAILODDS_API_KEY,
            'Content-Type'  => 'application/json',
        ],
        'body'    => json_encode(['email' => $email]),
        'timeout' => 5,
    ]);

    if (is_wp_error($response)) {
        return; // Fail-open
    }

    $result = json_decode(wp_remote_retrieve_body($response), true);

    if (isset($result['action']) && 'reject' === $result['action']) {
        $errors->add('validation',
            'This email address could not be verified. Please use a different email.');
    }
}, 10, 2);