Occasionally Outlook for Mac (OfM) will stop updating the Global Address List (GAL) from Microsoft Exchange Server. To fix this issue to the following:
Close OfM
From the Finder select the Go menu
Hold down the Option key and Library will appear
Select Library
Navigate to ~/Library/Caches/Outlook/Main Identity/#
Delete out all the folders under Main Identity
Open OfM, give it about 5 minutes to start downloading the GAL