class Mage_Core_Model_Email_Template ...

    public function send($email, $name = null, array $variables = array())
    {
        ...
        $setReturnPath = Mage::getStoreConfig(self::XML_PATH_SENDING_SET_RETURN_PATH);
        switch ($setReturnPath) {
            case 1:
                $returnPathEmail = $this->getSenderEmail();
                break;
            case 2:
                $returnPathEmail = Mage::getStoreConfig(self::XML_PATH_SENDING_RETURN_PATH_EMAIL);
                break;
            default:
                $returnPathEmail = null;
                break;
        }

        if ($returnPathEmail !== null) {
            $mailTransport = new Zend_Mail_Transport_Sendmail("-f".$returnPathEmail);
            Zend_Mail::setDefaultTransport($mailTransport);
        }
        ...