The GAL is a catalog of all users and their respective email addresses within an Exchange environment. A copy of the GAL is cached in the Offline Global Address List OAB inside of Outlook. Outlook clients utilize this particular OAB to perform address book queries against the GAL without introducing additional strain on the Exchange environment.
By default, Outlook clients update OABs every 24 hours. However, the Offline Global Address List can be manually updated should a user need the latest changes from the Exchange environment.
Open Outlook and select the Send / Receive tab.
Click on the Send/Receive Groups button and select Download Address Book….
In the Offline Address Book dialog, uncheck Download changes since last Send/Receive, select \Offline Global Address List under Choose address book:, and then click the OK button.
Unchecking Download changes since last Send/Receive ensures that the client downloads a fresh copy of the entire OAB.