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Outlook 2010, by default, now cache’s any additional mailboxes a user may have attached. This can cause the locally held OST file to jump in size quite considerably! I certainly tend to see issues on older machines once the OST file goes over 10GB but some have hit the 50GB mark which cripples Outlook – I believe this is also the physical cap for the file.

In Outlook 2007 you could turn caching on for shared mailboxes via this MS Article (HERE)

In Outlook 2010 you can turn this feature off so it reverts back to how Outlook 2007 handled cache via this MS Article (HERE)

To turn Outlook 2010 Cache for Shared Folders off:

  1. Close Outlook
  2. Open RegEdit (via Start, Run, type in Regedit, Click OK)
  3. Browse to the below key –
    HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Cached Mode
  4. Look for (or create) the following REG_DWORD –
    CacheOthersMail
  5. Set the Value to 0
  6. Job Done!

I’m not aware of any immediate Best Practices but as a possible work around (should users need an offline ability for any additional mailboxes) Outlook 2010 is now able to hold multiple Exchange accounts within one profile. These each then cache to their own individual OST files and show as if they were attached mailboxes to help remove any confusion to an end user.