The Global Address List (GAL) is a master list of all the users and their contact information within your organization. From time to time, you may face issues with Global Address List. One of the known problems with GAL is that its contacts do not update correctly. In other words, if you made certain changes to your GAL, they do not register on your Exchange server. As an administrator, you can fix this issue by "rebuilding" the address book.
Open "Exchange Management Shell." This is an administrative tool that is typically installed on a PC server computer. You will find this tool under the "Microsoft Exchange Server" folder on your "Start" menu.
Type "get-offlineaddressbook | update-offlineaddressbook" and press "Enter."
Let Exchange Management Shell "rebuild" your Global Address List. Once the process is complete, check your Address Book. You should be able to see the changes or updates you made on the Global Address List