unable to map subdomains to blog sites

I am running WP multisite 4.0.1 on a Windows 2003 IIS 6 dedicated server, so I can do everything I need on it.

I have installed the Domain Mapping plugin and I have been able to have it up and running (no alert on the dashboard of the plugin).

My WP main site is hosted on a domain http://www.mycompany.com.

I sell a product and for each of my customers, the product is hosted on mycustomerX.myproduct.com.

I want to provide a blog for each of my customer hosted on http://www.mycustomerX.myproduct.com.

To test this, I have defined a A record on subdomain http://www.mycustomer1.myproduct.com which redirects to the IP adress of the server.

When I go to the blog I want to access using this subdomain and try to assign the subdomain using the plugin panel, I get the following error: Domain name is unavailable to access.

So just for test purposes, I tested on another domain pointing on the same server and I am sure it is available, and get the same error.

So something is wrong in my settings.

Thanks in advance for tour feedback.