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 the Yesware for O365 add-in different from the Yesware for Outlook add-in? What versions of Outlook does Yesware support? How do I use Send with Yesware to track and sync my email? Re-enabling a disabled add-in for Outlook 2016, 2013, and 2010 How do I install the Yesware for O365 add-in?