While the computer industry generally spells it as email, most style references and dictionaries spell it as e-mail. Even Wiki covers this briefly:
http://en.wikipedia.org/wiki/Email

I think you can get away with either version, though I personally spell it email because its shorter and doesn't waste a space with that hyphen.

One other argument for e-mail may also come from the Oxford English Dictionary - which is listed at dictionary.reference.com.

... the word `emailed' is actually listed in the OED; it means "embossed (with a raised pattern) or perh. arranged in a net or open work". A use from 1480 is given. The word is probably derived from French `e'maille'' (enameled) and related to Old French `emmailleu"re' (network). A French correspondent tells us that in modern French, `email' is a hard enamel obtained by heating special paints in a furnace; an `emailleur' (no final e) is a craftsman who makes email (he generally paints some objects (like, say, jewelry) and cooks them in a furnace).