What is a Microsoft Office Add-In? Carolyn Chassaigne October 18, 2016 19:02 Updated Office Add-Ins extend the functionality of Microsoft Office Apps using standard web technologies. Office Add-Ins can be added to Office365 apps by visiting the Office Store. Related articles How is Email Tracking from Yesware add-in different from the Yesware for Outlook add-in? How do I use the Email Tracking from Yesware add-in to track my email? What is the Email Tracking from Yesware Add-in? What versions of Outlook does Yesware support? What are Yesware templates and how do I create them?