It is instructive to compare code from the previous edition of my book to see what changed between Outlook 2003 and Outlook 2007 as well as the changes to the add-in model between VSTO 2.0 and VSTO 3.0. This is a VSTO Outlook add-in that handles the ItemSend event and checks if there are more than 25 recipients to warn you that you are sending to a lot of people.
See original here:
Changes in the Add-In Model between VSTO 2.0 and VSTO 3.0