Each Outlook add-in defines the context in which it is available, including the types of items and if the user is reading or composing an item. The Outlook items that support add-ins include email messages, meeting requests, responses and cancellations, and appointments.
The web components all run in the context of a browser in a sandbox. For an Outlook add-in, Outlook reads the manifest and hooks up the specified controls in the UI, and then loads the JavaScript and HTML. Unlike COM add-ins, Outlook add-ins don't have any code physically installed on the user's device or Outlook client. Outlook add-ins are different from COM or VSTO add-ins, which are older integrations specific to Outlook running on Windows. Outlook add-ins are integrations built by third parties into Outlook by using our web-based platform.
