When you start typing an e-mail in Entourage and Mail.app, addresses popup that you have typed before some are not in your Address Book. Where do these e-mail addresses live?
Those email addresses live in a centralized database managed by the Mac OS called "The Truth". The Truth contains all of the contact and calendar information that is given to it by various applications such as Now X, Entourage, iCal, AddressBook, etc. which communicate through an Apple technology called Sync Services. NUDC5 doesn't support Sync Services and it would be difficult to do so, which is one of the reasons we wrote Now X.