Available public : List of exported Outlook APIs - definitions, data structures, functions & interfaces
As described in the MSDN article, Microsoft Office Outlook exports the following definitions, data structures, functions, and interfaces that were originally designed for internal use but are now exposed for public consumption.
Definitions: Constants for Exported Outlook APIs
Data structures: CONTAB_ENTRYID, RTF_WCSINFO, RTF_WCSRETINFO, TZDEFINITION, TZREG, TZRULE
Functions: GetDefCachedMode, GetDefCachedModeDownloadPubFoldFavs, HrCreateApptRebaser, HrGetAutoDiscoverXML, MAPICrashRecovery, RebaseTaskComplete, RebaseTaskProgress, WrapCompressedRTFStreamEx