Global address list is not updating on client
Remember, if you are looking at the Global Address List in Outlook then you are NOT looking at the Servers Global Address List*.
If you are using Outlook 2003 or 2007 you may be in "Cached Mode", and you are looking at a copy called the "Offline address book" This only gets updated Every 24 hours, and the copy on the server only gets updated every 24 hours at (by default).
Due to various synching schedules, it can take up to 48 hours before you could actually see a change in Outlook after a modification has been made on the Exchange server or in Active Directory.
In this guide, the relevant timings are explained and instructions are given in how you can directly force an update and resync, which can be very handy when troubleshooting or when you work in an Exchange environment which sees a lot of user mailbox mutations.
To resolve this you have two option, manually update the Address Book or customize the update schedule.
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In Cached Exchange Mode, Outlook 2007 & 2010 downloads a local copy of the Global Address List (GAL), but only updates it every 24 hours.
The default configuration for Exchange 2010 has the Address Book will update every 24 hours at 5am.
During this process all new email addresses are added to the Global Address List ready for when the clients next connect and refresh their copy of the address book.
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Synchronizing your Outlook (cached) address book with the Global Address book may fix these discrepancies.