Side effects of using cached mode with Outlook and Exchange 2003, and how it relates to the Offline Address Book

One behavior change that isn't well known to most people using Outlook and Exchange is that when you turn on cached mode on the Outlook client, your address book reverts to the Offline Address Book that is generated by Exchange/Outlook. By default the OAB is updated every 24 hours since the last time it was accessed.

Typically, that doesn't cause a lot of problems except when a new user is added or deleted from the GAL and all the cached Outlook clients still see the entries or don't see the entries that are now invalid, yet people running non-cached are perfectly fine. A lot of times you end up thinking that the RUS service hasn't done its job but it really has.

If you want your cached users to allows 'ping' to the live GAL instead of using the offline address book, use this registry key:

HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Cached Mode
Parameter: DownloadOAB
Type: REG_DWORD
Value: 0x00000000

One thing worth noting: If users go offline with this registry setting, they won't have a GAL to refer to anymore, so it is almost better to setup the OAB to update more often if your Active Directory isn't huge.

More background information on OAB can be found here:
http://support.microsoft.com/kb/841273