Most home and personal accounts do not use Microsoft Exchange. The Offline Address Book contains the properties of a user, such as e-mail addresses, that Microsoft Outlook requires in order to send an e-mail message and display information about the user.
By using the Offline Address Book, Outlook does not have to connect to Exchange in order to resolve names or open the records for each user.
In Cached Exchange Mode, Outlook 2007 & 2010 downloads a local copy of the Global Address List (GAL), but only updates it every 24 hours.
In order to reflect any changes or updates made to the Global Address List, you can force Outlook to download a new copy of the GAL.
So log in to OWA and test if you can see the new users there.
I’m giving it a 99% chance that you will (and if you don’t you have a serious problem at hand!
Synchronizing your Outlook (cached) address book with the Global Address book may fix these discrepancies.
Instances of mail that is bounced back to the sender may also be fixed.
In the Information to download section, select Full Details and in the Address Book section, choose \Global Address List. **NOTE** Please read this section carefully, or your update will not work!!
The key issue here is that there are new users added to the directory but they are not showing up in outlook.
Now, if you are like most organisations then outlook will be operating in cached mode.
In that case, you can completely reset it by deleting the files making up the OAB or by recreating your mail profile.
Rather than waiting 24 hours for Outlook to update the Offline Address Book on its own, you can force an update via the Send/Receive menu button.