This domain name can’t be used because it contains a protected word or inappropriate language.

I am trying to add the domain proposal-delivery.com to https://domains.live.com/manage/default.aspx and am getting the error: 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.

We are using this domain for email only - to send out service proposals.  Your assistance would be greatly appreciated.

Thank You,
Tony