Outlook 2007 Additional Mailbox or Shared Mailbox does not cache

We have migrated mailboxes to a hosted Exchange Server very far away from us (will take approximately 13 hours for me to take a flight there).  Anywhere, the migration went well and we don’t have much problem accessing our migrated mailboxes.  Accessing speed is okay for all except for those with additional mailbox configured in there Outlook 2007. 

By default Outlook 2007 does not cache additional or shared mailboxes even the primary mailbox is set to cached mode.  This is bad because these poor users now have to bear with the slow access across the WAN.  Fortunately there is a solution to end their suffering.  This requires Outlook 2007 post-SP1 (most should be at SP2) and registry hack.

The registry hack:

• Close Outlook 2007.
• Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook
• Add a new key Cached Mode under HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook
• Add a new DWORD value CacheOthersMail in the Cached Mode key.
• Set the data of the CacheOthersMail value to 1.

This is how it will look like.

Start Outlook 2007 and access the additional or shared mailbox and it will start to download all the emails.

The alternate registry key to enable cached mode for additional or shared mailboxes is HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\Cached Mode.  This alternate registry key can be configured via GPO by using custom .adm template.

For Outlook 2010, cached mode for additional or shared mailbox is enabled by default.

Sources:

• You cannot cache shared mail folders in Outlook 2007
• By default, shared mail folders are downloaded in Cached mode in Outlook 2010