How does Skype determine when registering a working domain or a public one?

Question

Skype

Good afternoon, colleagues.

Recently, it has become impossible to register new Skype users on one of our domains. Writes that this is a working domain and if you want to use Skype, then buy S4B.

So the question is: how did Microsoft understand that this is a working domain, and how to make sure that our other domains do not become working too?

Answer 1

On the domain also defines.
If you want more accurate information, then write the domain name

Answer 2

When registering on their sites, Melkosoft writes that it subsumes under public domains only the well-known outlook, hotmail, gmail, yahoo and the like, everything else is a "working domain" for them