An Outlook VSTO add-in extension is built using Visual Studio Tools for Office, which is an extension of the .NET framework. These add-ins are typically developed using C# or VB.NET.

VSTO add-ins provide a higher-level, managed code approach to building Outlook extensions. They provide a more developer-friendly environment compared to COM add-ins.

Additionally, VSTO Add-ins are suitable for developers who prefer working with managed code and want to leverage the .NET framework. They provide a more streamlined development experience and are often chosen for business productivity solutions.