Open a message you received which was sent to multiple recipients. Right-click to an empty space in the header pane and choose 'Customize'.

Drag and drop the 'Smart Reply' button to the header pane and close the 'Customize' window.

You should now see a 'Reply All' button.

At first, when I right-clicked an area at the top of an e-mail in order to customize all future e-mails to include "Reply All," I only saw 4 items to select from. Then, I tried right-clicking in a higher location and saw many more items to select from, but no "Smart Reply" icon to select. In addition, when I did drag and drop the "Reply All" icon I finally saw, the result wasn't what I expected; Reply All did add to the top (though in an upper right unwanted location) but, even worse, only for that one e-mail, not all future e-mails.

Ah, I hadn't quite realized what the result would be when adding "Reply All'" to the top of a previously-received e-mail that had been to multiple people. I expected to see "Reply All" in all my future outgoing e-mails, but it only appears for future incoming e-mails that are to multiple people. Makes sense! I'm not even sure anymore how OE used to be; maybe the action-type icons are static, indepedent of the type of e-mail (outgoing, incoming, in from multiple people, etc.). OE is starting to be a distant memory. Thanks again.