Why does Microsoft Office Outlook 2003 require users to connect in Cached Exchange Mode to take advantage of its Junk E-mail Filter?

Outlook 2003 supports two methods for filtering junk mail: the client-side Junk E-mail Filter and the spam confidence level (SCL) used by the Exchange Intelligent Message Filter (IMF) or third-party solutions (see "IMF Alternatives," June 2004, InstantDoc ID 42684, for examples). The Junk E-mail Filter DLL works only against messages in the local store (i.e., the local offline folder store—OST), so Cached Exchange Mode is a must because running Outlook in online mode might not produce an OST. Filtering methods that rely on SCLs have no such requirements. A message with an SCL that exceeds the IMF's Store threshold is automatically dumped into the mailbox's Junk Mail folder; messages with an SCL between the Store and gateway thresholds are left for filtering by the Junk E-mail Filter. For more information about this filter, see "Office 2003 SP1 Arrives," July 2004, InstantDoc ID 43656 or "Outlook 2003's Junk E-mail Filter," March 2004, InstantDoc ID 41655. For more information about the IMF, see "Microsoft's Intelligent Message Filter," December 2004, InstantDoc ID 44860 or "The Exchange Intelligent Message Filter," June 2004, InstantDoc ID 42682.