How to block outgoing mail?

A few days ago my boss told me he wants to block some people’s outgoing/incoming mail.

He want to restrict some users to have only internal mail, others may be able to send & receive mail to/from some internet addresses.

We are running Exchange Server 5.5, I know the way to block incoming mail using my firewall SMTP server, but I don?t know how to block the outgoing mail.

Any ideas?.

Thanks,
Mauricio.