I go along with Merriam-Webster’s in preferring the hyphenated version, but this isn’t about preference, it’s about priority. I would have assumed that the hyphen came first and then (as happens in the course of events, cf. “base-ball”) people began writing it as one word. Not so, according to the investigation undertaken by Avva, who’s irritated by the whole question but can’t resist looking into it, in the same way (he says) that you can’t help probing the toothache with your tongue. He has investigated the Google Groups Usenet archives and found that from the very early ’80s, when there was barely such a thing, both forms were in use, and both grew in popularity throughout the decade, so that the evolutionary hypothesis fails. His suggestion is that CompuServe’s 1979 introduction of what they called EMAIL (so called, he wonders, because program names couldn’t contain a hyphen?) established that form in the awareness of computer users, so that it was available alongside the more natural “e-mail” as both began to supplant the term “mail” (which was originally used for the electronic variety as well). He is continuing his researches and welcomes any corrections and commentaries; I too welcome comments from anyone knowledgeable about the history of usage in those dark ages of the internet.