… Exchange Client Extensions (ECEs) represent an extensibility feature introduced with the Microsoft Exchange client in 1995. The Exchange client was a 16-bit mail application running against the earliest versions of Exchange Server. ECEs must be written in native code, typically using C++ and relying heavily on the Messaging API (MAPI). When Outlook replaced the Exchange client, ECEs were used to extend Outlook 97-98 until COM Add-ins replaced ECEs in Outlook 2000 as the primary extensibility technology for Outlook.
ECEs will continue to operate as expected in Outlook 2007 and earlier. However, ECEs will not load in Outlook 2010. Outlook 2010 has converted its own ECEs such as Delegate Access, Deleted Items Recovery, Exchange Extensions commands, and Exchange Extensions property pages to native Outlook code