Unable to Create Apple ID using Work Email

I've seen some other threads where this same question was asked, but I don't think it was answered. I've been helping someone migrate away from using their work iPhone for personal life, and as part of that we removed their personal Apple ID from their work phone. However, it would not let us create a new Apple ID using their work email address. It says "Your Apple ID cannot end in @[employer].com. Choose a different email address." There is no existing Apple ID using this email address, and the warning does not seem to relate to that issue. The entire employer's internet domain is blocked.

We ended up generating a new Apple ID without an email address so it created an icloud email account instead. But I still want to know why we couldn't use their work email - does Apple allow some corporations to manage their own Apple ID accounts internally?