_getHeaders example

if ($count > 1) {
            // Multipart message; create new MIME object and boundary             $mime     = new Zend_Mime($this->_mail->getMimeBoundary());
            $boundary = $mime->boundary();
        } elseif ($this->_isMultipart) {
            // multipart/alternative -- grab boundary             $boundary = $this->_parts[0]->boundary;
        }

        // Determine recipients, and prepare headers         $this->recipients = implode(',', $mail->getRecipients());
        $this->_prepareHeaders($this->_getHeaders($boundary));

        // Create message body         // This is done so that the same Zend_Mail object can be used in         // multiple transports         $message = new Zend_Mime_Message();
        $message->setParts($this->_parts);
        $message->setMime($mime);
        $this->body = $message->generateMessage($this->EOL);

        // Send to transport!         $this->_sendMail();
    }