Searched Google for “email” and returned 802,000,000 results.
Searched Google for “e-mail” and returned 1,300,000,000 results. But, also got Google’s “Did you mean: email?” prompt, which suggests that Google, at least, has “email” as a preferred term with “e-mail” as a variant term.
I almost always write “email,” but when I think about it, putting in the hyphen makes sense to me. But then, our language is changing, and probably the version without the hyphen is already accepted as proper English.