Use different email address to access Microsoft Account, cant use my email address as an alias

Hi there,

    This is Raees Iqbal, and I have a problem with renaming my account primary alias. I've actually bought this domain 'steelbrain.com.pk' for my company and I'm using this domain name to host our company's personal email addresses with Microsoft Outlook.com , by updating mx records for the domain. I'm using Windows 8 Enterprise Edition with a ymail.com's email address for my Microsoft profile, but now when I've bought this domain and have setup email accounts also, I want to change my Microsoft primary account alias to another email address with steelbrain.com.pk domain name. But when in https://account.live.com -> Overview -> Account aliases -> 'Alias' (section below my previous ymail.com email address) -> Rename -> Yes rename my primary alias -> Use an existing email address as your primary alias,  I type my new email address with steelbrain.com.pk domain (hosted on Outlook.com) and click 'Rename my primary alias, I'm getting this error "You can't use @steelbrain.com.pk in your Microsoft account email address. Please try a different email address." with no explanation at all. Now, I would like someone to explain me the problem with it. Help will be appreciated and thanks for reading.

Regards,

Raees Iqbal