Which is the most "proper" way of writing it? When I'm being informal I always write "email", but what if you're writing something serious/official/important?
The OED lists email, with e-mail as an alternate spelling. It also has a seperate entry for e-mail as a verb, which is a bit odd. At any rate, it looks like there isn't really a standardized spelling, so pick email or e-mail depending on whether or not you feel like busting out the hyphen.
I know that it's not the correct spelling (neither in English nor in German nor in any other language afaik), but I prefer "eMail" as it reminds me of eBay and eMule and just looks cool.