Email Reactions is a new feature (in Outlook on the Web) that allows you to quickly give feedback on an email without doing a reply or reply to all… think of it as similar to the “Like” feature on Facebook. Here’s how it works…
Looking at an email in Outlook on the Web, I now see a smiley face icon that displays the reactions available (thumbs up, love, celebration, laughter, surprise, and sadness). I sent this email off to Sandra:

Sandra clicked on the Celebration icon when she got the email. In my copy of the email, I see that I had one Celebration reaction, and I can hover over that to see who sent it:

In my copy of the email, I clicked on the Love reaction and got this pop-up on my screen letting me know someone (me!) reacted to the email:

And now in my email, I see both reactions I received:

Again… this is only for Outlook on the Web. I can react to any email in Outlook on the Web, but only people who are also using Outlook on the Web will be able to see the reactions.