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
setMimeBoundary() method, as in the following example:
Ejemplo 45.9. Changing the MIME Boundary
$mail = new Zend_Mail();
$mail->setMimeBoundary('=_' . md5(microtime(1) . $someId++));
// build message...