You can catch a wide swath of those alias domains by the highly advanced method of "check the MX record and see if it matches a known disposable email service".

You can catch a wide swath of those alias domains by the highly advanced method of "check the MX record and see if it matches a known disposable email service". This only accounts for some low-hanging fruit, of course, but at the end of the day the user can almost certainly just sign up with a "legitimate" free email provider or spin up a receive-only mail server wherever and there's not much you can do about that.