This is getting really annoying, so I did some research. According to all the sources I could find, the proper form is e-mail (hyphenated), and at the beginning of a sentence, if you want to capitalize, it should be e-Mail (hyphenated with only M capitalized). It is a common name, not a proper name, so should not be capitalized in the middle of a sentence.
According to the style guides of “Get it Write”, IEEE, Webopedia, and the Associated Press, as well as the Merriam-Webster dictionary and the New Oxford American Dictionary, the proper spelling is e-mail. Some cite email as a possible alternative spelling.
One source stated that typical evolution of new terms removes all spaces and hyphens from the word, so email, online, and website are becoming the norm, although not official yet.
My conclusion is that although “e-mail” appears to be the correct form, “email” is also accepted, is shorter, and easier to capitalize at the beginning of a sentence. So I’m going to go with “email” as the officially correct spelling in my text moving forward.
As a side note, this means that I will treat website, webpage, and online as correct, single word nouns.