[judge2020]

They say this

> We asked them to change the domain to a public one, which Microsoft support denied. It must be technically possible, I guess, but here we are.

So I suppose the issue is: when example.com is a verified domain for some AAD organization, users with a live.com account ending @example.com cannot accept invites to become a guest of an external AAD directory (example2.com) due to this error message.

But i'm skeptical if this is MS's problem where they can't downgrade a domain to only allow registrations via `live.com`, or if Tutanota just wants to have the best of both worlds where some email accounts (maybe even employees' personal email accounts) can be manually AAD-registered to allow data governance within their Microsoft 365 organization, while everyone that's not an employee can still create a live.com account and won't have issues joining external AAD directories.