Two issues: It's not a standard, Microsoft just did this with the assumption that most people are using Outlook anyway. They didn't care how this would work for

Two issues: It's not a standard, Microsoft just did this with the assumption that most people are using Outlook anyway. They didn't care how this would work for everyone else. Secondly: What does it mean to get a reaction to an email? Can I interpret your "thumbs up" as a sign to go ahead with something, or is it just an "acknowledge, I got your email". If we more or less agree at this point that email are "for serious business" then we must treat it as such and provide clear and precise communication. Reactions are the opposite of that, they are more easily interpreted wrong, depending on context, culture or mood or the recipient. There might be a really good and reasonable use for "reactions" in emails, but I seriously doubt that Microsoft went all in and made their UI/UX experts do the research and those researchers came back with a clear answer that this was great and here's how to implement it. Given that this isn't even a standard I feel like it's something someone did on a Friday afternoon to show that you could done it. The piggy-backing on SMTP headers have all the hallmarks of a hack.