Along with other Microsoft Office 2000 applications, Microsoft Outlook 2000 allows you to enhance and extend the application using Component Object Model (COM) add-ins. A COM add-in is a dynamic-link library (DLL) that’s specially registered so that it can be loaded by an Office 2000 application. You can build a COM add-in with Microsoft Visual Basic version 5.0 or later, or any of the Office 2000 applications in the Office 2000 Developer. You can also create COM add-ins with Microsoft Visual C++ or Microsoft Visual J++.
The COMAddIns property of the Application object gives you access to the COM add-ins loaded by Outlook.
Learn more about customizing Outlook using COM add-ins.