We have an addin built in .Net using Outlook IAs on a dev machine that had outllok 2007. The addin installs a toolbar on the main explorer with a button. When the button is clicked a form loads with properties from the current selected mail in the explorer
and performs some action.
On the clicnt they are now upgrading to 2010, and the toolbar is not showing up. It does not look like the addin is even showing up anywhere.
We also trigger the same form when a reply action happens. But in 2010, nothing happens.