i agree reactions can be useful, but adding them to email the way Microsoft has is obnoxious for recipients using any client other than theirs. and, i think this is probably their intention: receiving an email reaction in a client that doesn't render it as a reaction feels wrong and MS probably hopes this will encourage some people to switch to using Outlook.

the right way to add reactions to email would be to make it opt-in (and also not a vendor-specific header but instead something which aims to become a standard): clients should only allow reactions to messages which contain a header signaling that the sender supports receiving them.