This page lists some of the Outlook add-ins that are available in the Office Store. These add-ins are available only for Outlook.com and Office 365 Exchange Online accounts. They work with Outlook 2013 and above as well as Outlook 2016 for Mac, Outlook on the web, and (eventually) will work in Outlook for devices.
These add-ins are listed in a small bar between the message header and the message, as seen in this screenshot. Some add-ins also add buttons to the ribbon.
Click the Store button to view all Outlook Apps.
Prior to 2016, these add-ins were called Office Apps.
Disable the Office 365 Store for Users
While the Office 365 Store offers some useful software, some administrators will want to prevent users from installing third-party Office add-ins on their own. These administrators can turn off the Office 365 Store tile in the Office 365 portal, under Service Settings, User Purchasing. Use Group policy to block apps in the Office desktop applications, as well as to limit users to apps in a trusted catalog.
The Store button can be removed from the ribbon using GPO.
HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\office\16.0\outlook\options\webext
DWORD: storebuttoninribbonhometaballowed
Value: 0