function wpmu_signup_user_notification( $user_login, $user_email, $key, $meta = array() ) {
  
/**
 * Filters whether to bypass the email notification for new user sign-up.
 *
 * @since MU (3.0.0)
 *
 * @param string $user_login User login name.
 * @param string $user_email User email address.
 * @param string $key        Activation key created in wpmu_signup_user().
 * @param array  $meta       Signup meta data. Default empty array.
 */
  if ( ! apply_filters( 'wpmu_signup_user_notification', $user_login, $user_email, $key, $meta ) ) {
    return false;
  }

  $user            = get_user_by( 'login', $user_login );
  $switched_locale = switch_to_locale( get_user_locale( $user ) );

  // Send email with activation link.
  $admin_email = get_site_option( 'admin_email' );
  if ( $admin_email == '' ) {
    $admin_email = 'support@' . $_SERVER['SERVER_NAME'];
  }
  $from_name       = get_site_option( 'site_name' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
  $message_headers = "From: \"{$from_name}\" <{$admin_email}>\n" . 'Content-Type: text/plain; charset="' . get_option( 'blog_charset' ) . "\"\n";
  $message         = sprintf(
    
/**
 * Filters the content of the notification email for new user sign-up.
 *
 * Content should be formatted for transmission via wp_mail().
 *
 * @since MU (3.0.0)
 *
 * @param string $content    Content of the notification email.
 * @param string $user_login User login name.
 * @param string $user_email User email address.
 * @param string $key        Activation key created in wpmu_signup_user().
 * @param array  $meta       Signup meta data. Default empty array.
 */
    apply_filters(
      'wpmu_signup_user_notification_email',
      /* translators: New user notification email. %s: Activation URL. */
      __( "To activate your user, please click the following link:\n\n%s\n\nAfter you activate, you will receive *another email* with your login." ),
      $user_login,
      $user_email,
      $key,
      $meta
    ),
    site_url( "wp-activate.php?key=$key" )
  );

  $subject = sprintf(
    
/**
 * Filters the subject of the notification email of new user signup.
 *
 * @since MU (3.0.0)
 *
 * @param string $subject    Subject of the notification email.
 * @param string $user_login User login name.
 * @param string $user_email User email address.
 * @param string $key        Activation key created in wpmu_signup_user().
 * @param array  $meta       Signup meta data. Default empty array.
 */
    apply_filters(
      'wpmu_signup_user_notification_subject',
      /* translators: New user notification email subject. 1: Network title, 2: New user login. */
      _x( '[%1$s] Activate %2$s', 'New user notification email subject' ),
      $user_login,
      $user_email,
      $key,
      $meta
    ),
    $from_name,
    $user_login
  );

  wp_mail( $user_email, wp_specialchars_decode( $subject ), $message, $message_headers );

  if ( $switched_locale ) {
    restore_previous_locale();
  }

  return true;
}