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IMHO the second problem goes deeper:

Sign In with Apple is allowing you to "create an account"(author's words) on @company.com, which should not be supported in the firat place. Instead, it should rely in a central directory controlled by company.com for authentication

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Apparently so do Google and Github according to the other comments in this thread. Seems like a potential design flaw in these SSO implementations.

Those with Apple Business or School Manager can now claim domain names which blocks sign ups under claimed domain names.

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