exchange 5.5 x.400 addresses

we implemented exhange 5.5 under Winnt 4.0. Echange has SP4 and Winnt has SP6

we give our intranet staff users x.400 email addresses so as not to allow them to email anyone in the Internet. However, even though they are using the x.400 addressesthey can still send and receive emails from the internet.

what seems to be wrong with our setup? any comments will be appreciated.