Here's a question that came up today.
How do you prevent an Exchange 2003 user from sending and receiving SMTP traffic through the internet, while still allowing them to receive emails from within the Exchange organization?
In the Exchange System Management tool, you have to add the account in question deny lists in two separate places.
The first place is Message Delivery in Global Settings of your organization. Here you go to Properties and select Recipient Filtering tab, add all SMTP addresses for accounts that you want to deny receipt of internet email.
Second place is in any Internet Mail SMTP Connectors you have for your organization. Go into the Properties for each connector and select the Delivery Restriction tab. Add the accounts in question to the Reject Messages From list.
Might take 15 minutes or so before the changes become active.