In IC a Person record is first set up – and they have a valid email address. A Person record is then used to promote them into a User – the Username on the user record can be an email address but it doesn’t have to be the same as the Person’s email (but normally is to keep things simple). If you enter a username that has already been used then you will receive and error asking you to enter a different value.
You can have more than 1 Person set up with the same email address, but each Username must be unique.
You don’t have to validate the Username so this can be a dummy email address, all notifications from the system will be sent to the email address provided in the Person record.