Zend_Mail::setReturnPath PHP Method

    public function setReturnPath($email)
    {
        if ($this->_returnPath === null) {
            $email = $this->_filterEmail($email);
            $this->_returnPath = $email;
            $this->_storeHeader('Return-Path', $email, false);
        } else {
            /**
             * @see Zend_Mail_Exception
             */
            require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Return-Path Header set twice');
        }
        return $this;
    }

 protected function _sendEmail(array $email, array $user, Zend_Mail_Transport_Abstract $transport)
 {
     if (!$user['email']) {
         return;
     }
     $phraseTitles = XenForo_Helper_String::findPhraseNamesFromStringSimple($email['email_title'] . $email['email_body']);
     /** @var XenForo_Model_Phrase $phraseModel */
     $phraseModel = XenForo_Model::create('XenForo_Model_Phrase');
     $phrases = $phraseModel->getPhraseTextFromPhraseTitles($phraseTitles, $user['language_id']);
     foreach ($phraseTitles as $search => $phraseTitle) {
         if (isset($phrases[$phraseTitle])) {
             $email['email_title'] = str_replace($search, $phrases[$phraseTitle], $email['email_title']);
             $email['email_body'] = str_replace($search, $phrases[$phraseTitle], $email['email_body']);
         }
     }
     $mailObj = new Zend_Mail('utf-8');
     $mailObj->setSubject($email['email_title'])->addTo($user['email'], $user['username'])->setFrom($email['from_email'], $email['from_name']);
     $options = XenForo_Application::getOptions();
     $bounceEmailAddress = $options->bounceEmailAddress;
     if (!$bounceEmailAddress) {
         $bounceEmailAddress = $options->defaultEmailAddress;
     }
     $toEmail = $user['email'];
     $bounceHmac = substr(hash_hmac('md5', $toEmail, XenForo_Application::getConfig()->globalSalt), 0, 8);
     $mailObj->addHeader('X-To-Validate', "{$bounceHmac}+{$toEmail}");
     if ($options->enableVerp) {
         $verpValue = str_replace('@', '=', $toEmail);
         $bounceEmailAddress = str_replace('@', "+{$bounceHmac}+{$verpValue}@", $bounceEmailAddress);
     }
     $mailObj->setReturnPath($bounceEmailAddress);
     if ($email['email_format'] == 'html') {
         $replacements = array('{name}' => htmlspecialchars($user['username']), '{email}' => htmlspecialchars($user['email']), '{id}' => $user['user_id']);
         $email['email_body'] = strtr($email['email_body'], $replacements);
         $text = trim(htmlspecialchars_decode(strip_tags($email['email_body'])));
         $mailObj->setBodyHtml($email['email_body'])->setBodyText($text);
     } else {
         $replacements = array('{name}' => $user['username'], '{email}' => $user['email'], '{id}' => $user['user_id']);
         $email['email_body'] = strtr($email['email_body'], $replacements);
         $mailObj->setBodyText($email['email_body']);
     }
     if (!$mailObj->getMessageId()) {
         $mailObj->setMessageId();
     }
     $thisTransport = XenForo_Mail::getFinalTransportForMail($mailObj, $transport);
     try {
         $mailObj->send($thisTransport);
     } catch (Exception $e) {
         if (method_exists($thisTransport, 'resetConnection')) {
             XenForo_Error::logException($e, false, "Email to {$user['email']} failed: ");
             $thisTransport->resetConnection();
             try {
                 $mailObj->send($thisTransport);
             } catch (Exception $e) {
                 XenForo_Error::logException($e, false, "Email to {$user['email']} failed (after retry): ");
             }
         } else {
             XenForo_Error::logException($e, false, "Email to {$user['email']} failed: ");
         }
     }
 }