BES doesn't anyhow probe the other directories in the Domino environment unless you have configured its Domino server that host it to do so. By default the only Domino Directory (DA) known to BES i.e. NAMES.NSF. If you have Directory Assistance setup for other directories, BES searches those as well via the Notes API call for looking up contacts.
You should check in your Domino environment if:-
(1) DA has been setup
(2) User the Domino console command: show xdir
and see how many directories are made known to BES
You will find your answers with the above.
Source: Lotus Domino Administrator 6 Help - Show Xdir