public function addHeader($header, $value, $overwrite = null)
    {
        $lc_header = Horde_String::lower($header);

        if (is_null($overwrite) &&
            in_array($lc_header, $this->_headers->singleFields(true))) {
            $overwrite = true;
        }

        if ($overwrite) {
            $this->_headers->removeHeader($header);
        }

        if ($lc_header === 'bcc') {
            $this->_bcc = $value;
        } else {
            $this->_headers->addHeader($header, $value);
        }
    }