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:
Example #1 Changing the MIME Boundary
$mail = new Zend_Mail(); $mail->setMimeBoundary('=_' . md5(microtime(1) . $someId++)); // build message...