Managing personal Outlook.com account settings, security, and privacy
That is an odd error because the msn accounts are part of outlook.com (I have an msn address and have not seen that message) - unless you are getting it when you use the msn website. Log into outlook.com using your address. You can add an outlook.com alias to the account at https://account.live.com/names/manage but don't Remove the msn address or you will lose it forever.