uc_signup only checks for valid_email_address() in allowing a user to sign up through ubercart, not drupal_is_denied()...

Easy option is to put drupal_is_denied() in, but I do wonder if we should actually allow blocked email addresses to be attendees, but not to register on the site. Or at least have the option.

For example, possible use case is a drupal site restricted to an organisational email address suffix... but wants to allow external users to buy signups. In these cases it would would mean preventing the triggered process of registering new email addresses as users onto the system, though.