tags.
      if ($email['body'] === strip_tags($email['body'])
        && preg_match('/.' . $eol . './', $email['body'])
      ) {
        // In condition:
        // No html tags.
        // At least one embedded newline.
        $email['body'] = '
' . $email['body'] . '
';
      }
    }
    // Theme with htmlmail.tpl.php.
    $body = theme('htmlmail', $email);
    $mime->setHTMLBody($body);
    // @todo Change to drupal_html_to_text when issue #299138 gets resolved.
    module_load_include('inc', 'mailsystem', 'html_to_text');
    $mime->setTXTBody(mailsystem_html_to_text($body));
    $message['MailMIME'] = &$mime;
    return $body;
  }

  /**
   * Send an email message.
   *
   * @param array $message
   *   An associative array containing at least:
   *   - headers: An associative array of (name => value) email headers.
   *   - body: The text/plain or text/html message body.
   *   - MailMIME: The message, parsed into a MailMIME object.
   */
  public function mail(array $message) {
    $eol = variable_get('mail_line_endings', MAIL_LINE_ENDINGS);
    // Ensure that subject is non-null.
    $message += array('subject' => t('(No subject)'));
    // Check for empty recipient.
    if (empty($message['to'])) {
      if (empty($message['headers']['To'])) {
        watchdog(
          'HTMLMailSystem',
          'Cannot send email about %subject without a recipient.',
          array('%subject' => $message['subject']),
          WATCHDOG_ERROR
        );
        return FALSE;
      }
      $message['to'] = $message['headers']['To'];
    }
    if (class_exists('MailMIME')) {
      $mime = new MailMIME();
      $to = $mime->encodeHeader('to', $message['to']);
      $subject = $mime->encodeHeader('subject', $message['subject']);
      $txt_headers = $mime->txtHeaders($message['headers']);
    }
    else {
      $to = mime_header_encode($message['to']);
      $subject = mime_header_encode($message['subject']);
      $txt_headers = $this->txtHeaders($message['headers']);
    }
    $body = preg_replace('#(\r\n|\r|\n)#s', $eol, $message['body']);
    // Check for empty body.
    if (empty($body)) {
      watchdog(
        'HTMLMailSystem',
        'Refusing to send a blank email to %recipient about %subject.',
        array('%recipient' => $message['to'], '%subject' => $message['subject']),
        WATCHDOG_WARNING
      );
      return FALSE;
    }
    if (variable_get('htmlmail_debug', 0)) {
      $params = array(
        $to,
        $subject,
        drupal_substr($body, 0, min(80, strpos("\n", $body))) . '...',
        $txt_headers,
      );
    }
    if (isset($message['headers']['Return-Path'])) {
      // A return-path was set.
      if (isset($_SERVER['WINDIR']) || strpos($_SERVER['SERVER_SOFTWARE'], 'Win32') !== FALSE) {
        // On Windows, PHP will use the value of sendmail_from for the
        // Return-Path header.
        $old_from = ini_get('sendmail_from');
        ini_set('sendmail_from', $message['headers']['Return-Path']);
        $result = @mail($to, $subject, $body, $txt_headers);
        ini_set('sendmail_from', $old_from);
      }
      elseif (ini_get('safe_mode')) {
        // If safe mode is in effect, passing the fifth parameter to @mail
        // will cause it to return FALSE and generate a PHP warning, even
        // if the parameter is NULL.
        $result = @mail($to, $subject, $body, $txt_headers);
      }
      elseif ((variable_get('site_mail', ini_get('sendmail_from')) === $message['headers']['Return-Path'] || self::_isShellSafe($message['headers']['Return-Path']))) {
        // On most non-Windows systems, the "-f" option to the sendmail command
        // is used to set the Return-Path.
        $extra = '-f' . $message['headers']['Return-Path'];
        $result = @mail($to, $subject, $body, $txt_headers, $extra);
        if (variable_get('htmlmail_debug', 0)) {
          $params[] = $extra;
        }
      }
    }
    else {
      // No return-path was set.
      $result = @mail($to, $subject, $body, $txt_headers);
    }
    if (!$result && variable_get('htmlmail_debug', 0)) {
      $call = '@mail(' . implode(', ', $params) . ')';
      foreach ($params as $i => $value) {
        $params[$i] = var_export($value, 1);
      }
      if (defined('DEBUG_BACKTRACE_IGNORE_ARGS')) {
        $trace = print_r(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS), 1);
      }
      else {
        $trace = debug_backtrace(0);
        for ($i = count($trace) - 1; $i >= 0; $i--) {
          unset($trace[$i]['args']);
        }
        $trace = print_r($trace);
      }
      watchdog('htmlmail', 'Mail sending failed because:
@call

returned FALSE.
@trace
', array('@call' => $call, '@trace' => $trace));
    }
    return $result;
  }

  /**
   * Converts an array of email headers to a text string.
   *
   * @param array $headers
   *   An associative array of ('HeaderName' => 'header value') pairs.
   *
   * @return string
   *   The concatenated headers as a single string.
   */
  public function txtHeaders(array $headers) {
    $output = array();
    foreach ($headers as $name => $value) {
      if (is_array($value)) {
        foreach ($value as $val) {
          $output[] = "$name: $val";
        }
      }
      else {
        $output[] = "$name: $value";
      }
    }
    return implode("\n", $output);
  }

  /**
   * Disallows potentially unsafe shell characters.
   *
   * Functionally similar to PHPMailer::isShellSafe() which resulted from
   * CVE-2016-10045. Note that escapeshellarg and escapeshellcmd are inadequate
   * for this purpose.
   * This method is copied from Core's DefaultMailSystem class.
   *
   * @param string $string
   *   The string to be validated.
   *
   * @return bool
   *   True if the string is shell-safe.
   *
   * @see https://github.com/PHPMailer/PHPMailer/issues/924
   * @see https://github.com/PHPMailer/PHPMailer/blob/v5.2.21/class.phpmailer.php#L1430
   *
   * @todo Rename to ::isShellSafe() and/or discuss whether this is the correct
   *   location for this helper.
   */
  protected static function _isShellSafe($string) {
    if (escapeshellcmd($string) !== $string || !in_array(escapeshellarg($string), array("'$string'", "\"$string\""))) {
      return FALSE;
    }
    if (preg_match('/[^a-zA-Z0-9@_\-.]/', $string) !== 0) {
      return FALSE;
    }
    return TRUE;
  }

}