Managing personal Outlook.com account settings, security, and privacy
Did you use the alias to send an email or log into any other web site? The trick is to use the alias ONLY to log in and to NEVER use it for anything else. If you use it to send an email or log into some other web site, the alias is then exposed to the world, the hackers pick up on it, and try to log in with it. So, create another new alias, aet it as the login address, and NEVER use it for anything else.