Allow Contacts to be Hidden from GAL
Allow contacts (from outside the domain) to be hidden from the address book. It sounds counter-productive, but I ran into a situation where I wanted to add an outside user to a distribution list, but not show in the GAL. The user had both an internal and external email address and I didn't want to confuse users.
Drew Lanclos commented
This can be done in PowerShell with Set-MailContact -HiddenFromAddressListsEnabled $true.
If it's a dirsync'd contact, then it will also respect msExchHideFromAddressBook (or whatever the property is).
It should be visible in the UI of course.