Original title: This domain name can't be used because it contains a protected word or inappropriate language. Please contact support if you feel this is not the case.

I have a domain that I would like to transfer over to Microsoft.  It's cumulus10.com, which is a personal website.      Cumulus is a type of cloud, so this name is a whimsical version of "Cloud 9" or "Cloud 10" which means a happy experience.

Here is the message I get:
This domain name can't be used because it contains a protected word or inappropriate language. Please contact support if you feel this is not the case.

I'm not sure why that domain name is protected or inappropriate.  Can you help?