In the exercise 'validating user entries', the standard validation rule, ValidEmailAddress, that is shipped as part of the product must be used.
However, when testing my rule, valid e-mail addresses got rejected.
(for example, an address containing the DNS root, expressed as an empty label followed by a dot '.').
There may be good reasons to do that, but please call such a validation rule 'StandardEmailAddress' or something like that, instead of ValidEmailAddress. Or alternatively, consider adhering to the standards (RFC's).