In a multipart message, a MIME boundary for separating the different parts of the message is normally generated at random. In some cases, however, you might want to specify the MIME boundary that is used. This can be done using the setMimeBoundary() method, as in the following example:

$mail = new Zend_Mail();
$mail->setMimeBoundary('=_' . md5(microtime(1) . $someId++));
// build message...