I get told that my email is invalid by some apps, for using a .so domain. Apple was one that would not let me create an apple ID upon buying a new laptop. Eventually, I set up the machine without Apple ID, and opted to create it after the fact via apple.com which accepted it. It makes me wonder if it is intentionally a "Hard no" during system setup, and then a "well okay if you insist" when the user has installed the system without using an Apple ID as a last resort.