You can set this globally, for example in your bootstrap.php file:
$tr = new Zend_Mail_Transport_Smtp('smtp.gmail.com', array(
'auth' => 'login',
'username' => 'YOUR_USERNAME@gmail.com',
'password' => 'YOUR_PASSWORD',
'ssl' => 'ssl',
'port' => 465)
);
Zend_Mail::setDefaultTransport($tr);