Actually I just figured it out.

I needed to add one extra line to your code for authenticated SMTP (highlighted in red):
Code:
        Zend_Loader::loadClass('Zend_Mail');
        Zend_Loader::loadClass('Zend_Mail_Transport_Smtp');
        $config = array('ssl' => 'tls', 'port' => 587, 'auth' => 'login', 'username' => '[email protected]', 'password' => '123abc');
        $tr = new Zend_Mail_Transport_Smtp('smtp.domain.com', $config);
Thank you so much for your help! This is resolved now.