If you attempt account creation with a non-allowed domain when a signup rule exists, the error message you see is "email is not defined".
This error message isn't helpful or instructive for users who perhaps just need to try their work email rather than their personal one.
It would be great if this "email is not defined" message when account creation is attempted with a non-allowed domain were one of the customizable error messages. For our use case as an edtech company, this would be something like "Looks like you tried to login with an email domain we don't recognize. Enter your school email and try again"