The shoulda woulda here is the OP setting up the aliases as separate accounts in the beginning. Each major free email provider has it's own set of features and doesn't really represent that the features will work in any email client, future or present, that the user may try.
With Outlook.com email they've got you snookered with aliases. From what I understand if you delete an alias and wait 30 days a new account with that address can be created…by anyone I presume.
Too bad you can't, on the Outlook.com website, select an email alias and push a "Create New Account With This Email Address" button.
If we're talking about the free Outlook.com service here I guess the user gets what he/she paid for.