I working on building an add-In for outlook.com (web version). It perform some action on emails. Currently it works with single email i.e. when you are reading an email, i click on the "..." menu and i can see my add-In there.
However i am facing 2 issues:
what I know so far is, "ExtensionPoint" is the property that defined where add-in will apear which currently can be set as MessageReadCommandSurface or MessageComposeCommandSurface but this display add-in on single email level
You can pin the add-in to Message window (quick actions) using
settings -> Mail -> Customize actions -> Message Surface/Toolbar
You can choose the surface (based on add-ins availability on that surface) to show the add-in individually. Visit here to find more.
Currently the feature of enabling add-ins on multiple selected mails is not there. We track Outlook add-in feature requests on our user-voice page. Please add your request there. Feature requests on user-voice are considered, when we go through our planning process.