Are you interesting in Outlook 2013 development? Then this new MSDN section is for you: Mail apps for Outlook. Make sure you also check the new Apps for Office and SharePoint Blog.
Overview
This section contains conceptual information specific to building a mail app for Outlook and Outlook Web App Preview for Exchange 2013 Preview.
Seeing mail apps in Outlook
The following topics answer the basic question “What is a mail app?” from both the end user and developer perspectives. They point you to videos of sample mail apps running in Outlook, and also explain the sample code.