Someone sent me two calendar invites from Outlook, but was send to an email address that is one of my Microsoft account aliases.

When I try to add these calendars to Outlook it askes for my ID and password. I try to use the alias email address as my ID but it doesn't accept it. I also try to use my primary emails address, but that was not being accepted either.

Is it only web services that can use any alias or am I missing something here?