Some users or distribution groups need to be hided from Address list some reason. You have two options to hide.
First option is always GUI
Second option is Exchange Server Powershell
set-mailbox –hiddenFromAddresslistEnabled $true –Identity “Sonat.YAYLALI”
Let’s check it on GUI and Powershell
get-Mailbox –Identity “Sonat YAYLALI”
HiddenFromAddressListsEnabled : True
Before checking it on GUI, you should refresh EMC