I've seen this question referenced before but have not found an answer. How do you hide an email address from the Global Address List? I'm running Exchange 5.5.
In the Exchange administrator application, locate and open the properties of the user you want
to hide. Navigate to the advanced tab and you'll see a checkbox for "hide from address book." Check
the box and click OK to save your changes, and the recipient will be hidden. Note that this hides
the entire recipient, not just one of his or her email addresses.
This was first published in September 2003