Gosh, what IS IT with people and these little fit-and-finish threads!? It's maddening when real, should-be usable features... like S/MIME!! receive virtually zero attention from either the user base or the Windows Central staff!!
I visited my local Microsoft store; they couldn't answer the question and I played with the OS, myself, and I could not find an email encryption/digital signature setting for an Exchange-configured account. It's feeling like S/MIME, like so many other features of the Windows 10 platform, has simply taken a back seat to a plethora of other bugs yet to be worked out in an OS released ahead of being ready. Too bad because enthusiast or not, if S/MIME is a feature you rely upon daily--I do!!--you aren't going to invest in the new hardware, no matter how much you might be wanting to.