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