RESTRICTING EMAIL SENDING IN EXCHANGE 5.

I’m runing exchange server 5.5 in a organization with 1,000 clients. All the departmens can send mails one to another, but sending mails to the global address list is not “permited” by internal regulations.

Some users are still sending mails to the Global Address List. The question is: How can i prevent users from sending mails to everyone, without blocking them to send mail to one or more users ???