As already reported in issue ZF-6219, the character set in the Content-type header of MIME message part should not be quoted.

Content-Type: text/html; charset="utf-8"

The solution that was proposed there was to remove the quotes in Zend/Mime/Part.php which was done in the next release i think. The problem now is that the mentioned file is not the only one that quotes the mail headers that way. Another file that always set my headers wrong when I was setting an attachment:

Mail/Transport/Abstract.php (line 143):

$this->_headers['Content-Type'] = array(
      $type . '; charset="' . $this->_mail->getCharset() . '";'
        . $this->EOL
                . " " . 'boundary="' . $boundary . '"'
        );

Removing the doublequotes on the charset= part lets even outlook 2003 read my attachment properly =).