Last year I created a tutorial on how to setup custom domain email address on outlook.com. Unfortunately, the free service is now no longer supported.
Users are encouraged to migrate to Office 365 - a Microsoft's premium online service which includes enterprise-class email service - to continue manage their custom domains.
If you already setup a custom domain email address on outlook.com, you don't have to worry though. You still can access your email like usual, as stated on the page:
If you previously used custom domains with Outlook.com, we still support the ability to log in to Outlook.com with your existing custom domain email address, but admins can no longer manage accounts in the domain. These accounts are still able to send and receive email, and you don’t need to make any changes.
This is the official announcement on Windows Live Admin Center page: