Every exchange admin would have come across a user complaining that the new staff is not present in the address book and hence he/she can’t email the new member of staff.
There are number of ways of solving this and I am sure you are all aware of it.
The Outlook cached mode feature was designed to reduce network traffic between the Outlook clients and the Exchange server.


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Visit Stack Exchange When a new email address is added to our company's Exchange server it doesn't show up immediately on my Outlook, and I suspect that it's because of the "cached mode".
The offline address book contains the properties of a user, such as email addresses, that Microsoft Outlook requires to send an email message and display information about the sender.
By using an offline address book, Outlook doesn’t have to connect to Microsoft Exchange Server to resolve names or open the records for each user.
It should be noted that mobile users should be avoided from this registry edit as they won’t have an address book otherwise (while working offline)!
HKEY_CURRENT_USERSoftware Microsoft Office14.0Outlook Cached Mode Parameter: Download OAB Type: REG_DWORD Value: 0 HKEY_CURRENT_USERSoftware Microsoft Office12.0Outlook Cached Mode Parameter: Download OAB Type: REG_DWORD Value: 0 HKEY_CURRENT_USERSoftware Microsoft Office11.0Outlook Cached Mode Parameter: Download OAB Type: REG_DWORD Value: 0 Setting the value to zero prevents OAB download and forces Outlook to use the global address list. Note If files were previously downloaded and associated with an Outlook profile, this setting will not have the correct effect.
If Outlook is left running constantly in Cached Exchange Mode, it automatically updates the offline address book about one time per day depending on updates on the computer that is running Exchange.
The offline address book size can vary from several megabytes to hundreds of megabytes in uncompressed form.
To use this setting, you must set the registry value, remove any files from the local computer and then synchronize. Cloud Architect & Blogger with interests in Office 365, Enterprise Mobility & Security and Azure.
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It does this by creating a local copy of the mailbox and synchronising the updates as needed.