Using Word XP (2002) I discovered that according to Microsoft there are two correct choices Email or e-mail. Since we know that Bill Gates rules the world, this must be, in fact, the correct answer.
So, if you don't want to use the hyphen, then you must capitalize the letter "E."
Don't bother to disagree, :argue because if Microsoft says this is how it should be done, sooner or later you will be forced to adjust. :sc
I think I have probably used almost every single version of spellings of this noun that you all have listed at least one time or another. I must say that I agree, especially in this case, with the bumper-sticker which says-
Why limit our creative communication ability with only one or two or three correct spellings?