Windows Mail (formerly Outlook Express) Disclaimer. New applications should not use this technology. It exists for backward compatibility with legacy applications. This technology may not be supported in the next version of Windows. Introduction Windows Mail (formerly Outlook Express) is an online communication tool supplied as part of Windows Vista and later versions. This documentation is provided for programmers who want to extend the functionality of Windows Mail. Developer Audience. The Microsoft Visual C++ class-based interface is designed for use by C/C++ programmers. Familiarity with Component Object Model (COM) is required. Additionally, familiarity with the Windows graphical user interface and message-driven architecture may be helpful. Windows Mail Documentation