How do I use TLS email with web.config

Here is my web.config file:

<system.net>
    <mailSettings>
        <smtp from="xxx@gmail.com"  >
            <network host="smtp.gmail.com" port="587" userName="xxx@gmail.com" password="yyy" />
        </smtp>
    </mailSettings>
</system.net>

I need to enable TLS, a requirement of my email server. However I only see SSL.