public function getHeaderContent()
    {
        $headerContent = '';
        $sender = $this->getSender();
        if (!empty($sender)) {
            $headerContent .= 'From: ' . $sender . PHP_EOL;
        }
        $reply = $this->getReply();
        if (!empty($reply)) {
            $headerContent .= 'Reply-to: ' . $reply . PHP_EOL;
        }
        $recipientCc = $this->getRecipientCc();
        if (!empty($recipientCc)) {
            $headerContent .= 'Cc: ' . $recipientCc . PHP_EOL;
        }
        $recipientBcc = $this->getRecipientBcc();
        if (!empty($recipientBcc)) {
            $headerContent .= 'Bcc: ' . $recipientBcc . PHP_EOL;
        }
        if (!empty($this->mailer)) {
            $headerContent .= 'X-Mailer: ' . $this->mailer . PHP_EOL;
        }
        $headerContent .= 'MIME-Version: 1.0' . PHP_EOL;
        if (!empty($this->attachmentList)) {
            $contentType = 'multipart/mixed';
        } else {
            $contentType = 'multipart/alternative';
        }
        $headerContent .= 'Content-Type: ' . $contentType . '; boundary="' . $this->boundary . '"';
        $headerContent .= PHP_EOL . PHP_EOL;
        return $headerContent;
    }