When we add a new user account in Active Directory (AD), the user sometimes isn't visible in the Outlook address book until the next day. What's going on?

You're probably running into this problem because of the way Outlook and Exchange Server cooperate to handle the offline address book (OAB). Outlook 2000 and later attempt to resolve names from the OAB, if it's present. If Outlook 2002 or Outlook 2000 can't find a name in the OAB, it will send the name to the Exchange server (for Outlook 2000) or Global Catalog (GC) server (for Outlook 2002 and later) for resolution. But if the user is running Microsoft Office Outlook 2003 in Cached Exchange Mode, the lookup will fail until the OAB is updated. Outlook 2003 downloads the full OAB when you first install it; after that, it downloads smaller updates that contain only changes that have occurred since the previous update, at a default interval of 24 hours. You can't control when Outlook will attempt to download the OAB, but you can control how it downloads full and differential updates. (The Microsoft article "How to configure how the Offline Address Book is downloaded when you use Outlook 2003 in Cached Exchange Mode" at http://support.microsoft.com/?kbid=823580 explains how.) Or read the Microsoft article "How to Force Outlook to Resolve Proxy Addresses and Custom Properties in Cached Mode" (http://support.micro soft.com/?kbid=831124), which explains how you can make Outlook always resolve names against the Global Address List (GAL). Be forewarned: If you make the change that this article describes, users might not be able to resolve names while working offline. You can also change Exchange's OAB regeneration schedule, although you can't force the client to synchronize the OAB at a particular time.