To be frank I quite like this idea. Can’t we standardize it somehow and add it to other email clients too? IMO the fallback, that internally it’s just a regular, human-readable email, is quite neat (though I wonder how they dealt with other languages than English - do they know which language the email is in? Is it just always English? That’d be bad).

I work on a chat component library (https://talkjs.com) and we support both emoji reactions and email notifications for missed chat messages. These emails can be replied to and they show up as chat messages in the conversation. It’d be very natural for us to add reaction support to the emails too, but I’d be reluctant to do so if it means a great UX for Outlook users but a terrible UX (overload of little reaction emails) for the rest.