We have SBS 2003. We have several domains registered and each user, when they are created, gets an email address on each one of these domains (listed in the "email address" tab in the active directory properties box.) For every user, the default address works fine (firstname.lastname@example.org)
However, some users cannot receive mail at one of the other domains. For example, email@example.com. For other users, though, they can receive mail sent to every one of the domains, so I know the domain isn't the problem-- it must be something with the individual user account / settings.