Hide Contacts from Showing Up in GAL

In certain environments, you may have created contacts which you don't want to show up in your Global Address List (GAL).

To hide these certain contacts,

1. Open Active Directory Users and Computers.
2. Locate the contact that you want to be hidden.
3. Open their properties and go to the Exchange Advanced tab.
4. Tick the box that reads "Hide from Exchange address lists".

In my case, it took about 5 minutes for the contact to be hidden in the GAL. If you're using Outlook in cache mode, it may take up to 24 hours for this change to take effect.