Microsoft 365 Domains Cannot Create Microsoft Accounts

October 2016 — If you have a domain domiciled in an Microsoft 365 tenant, Microsoft will not allow the creation of standalone Microsoft accounts:

As of November 2016, the language on the site has been amended to “You can't sign up here with a work or school email address. Use a personal email, such as Gmail or Yahoo!, or get a new Outlook email” to better reflect the restriction.