To send mail via SMTP, Zend_Mail_Transport_Smtp needs to be created and registered with Zend_Mail before the send() method is called. For all remaining Zend_Mail::send() calls in the current script, the SMTP transport will then be used:

<?php
require_once 'Zend/Mail/Transport/Smtp.php';
$tr = new Zend_Mail_Transport_Smtp('mail.example.com');
Zend_Mail::setDefaultTransport($tr);
?>   

The setDefaultTransport() method and the constructor of Zend_Mail_Transport_Smtp are not expensive. These two lines can be processed at script setup time (e.g., config.inc or similar) to configure the behaviour of the Zend_Mail class for the rest of the script. This keeps configuration information out of the application logic - whether mail is sent via SMTP or mail(), what mail server to use, etc.