works fine for flat arrays but for nested arrays (nested multipart bodys specifially) requries iterative reduction.

Recommendation: class Zend_Mail_Part line: c. 360

public function _header_join($txt, $item) {
    if ($txt) {
        $txt.= Zend_Mime::LINEEND;
    }
    if (is_array($item)) {
        $item = array_reduce($item, array($this, '_header_join'), '');
    }
    $txt .= $item;
    return $txt;
}

    switch ($format) {
        case 'string':
            if (!$header || (!count($header))){
                $header = '';
            } elseif (is_array($header)) {
                $header = array_reduce($header, array($this, '_header_join'), '');
            }
            break;
        case 'array':
            $header = (array)$header;
        default:
            // do nothing
    }