At times IIS 7.x needs to accept the domain before it can process your web.config file.
From your IIS Manager (Internet Information Services Manager)
Click on your website.
To the right in the Action column find "Edit Site" below that you'll see Bindings.
In Bindings > you will see "Type" , "Hostname", "Port", "IP Address", "Binding Information".
You will see your http, domain.ext, 80, 10.0.0.1, Information.
Add another entry: http, www.domain.ext, 80, 10.0.0.1, information.
To force good SEO you can create your redirect rules to redirect domain.com to www.domain.com.
Be sure to add or remove the trailing slash as well if do not redirect to a static page.