Using New Outlook on Windows for professional communication and productivity
It looks like Microsoft wants every Microsoft 365 email account to be licensed for desktops apps in order to work on new Outlook.
On the old desktop Outlook as long as one account is licensed you can use the app for any other account.
This creates the absurd situation where Microsoft have created a version of Outlook where Apple Mail works but not their own paid-for email service.
Companies with more email accounts than desktop app users (e.g. with 'sales' or 'info@' email accounts) are also stung because this set up means a company managing multiple inboxes will now needs to be licensed for desktop apps for each account even though nobody is using the apps.
This isn't going to work. You can't force people to pay extra for what they already had.