I wouldn't worry about using the prevalent pluralization of e-mail (or email) "e-mails," as it's so commonly used and widely recognized. The same for "an e-mail." E-mail has become a noun of its own, independent of its original form, "electronic mail."

That being said, if you *do* wish to use e-mail as a derivative of mail (probably only to be considered when used as the formal, perhaps even archaic "electronic mail"), I suppose it would make sense to go by mail's rules. But you wouldn't need to say "I received a piece of email from John" (as this wouldn't make much sense), just "I received email from John." Though neither would you need to say "I received a piece of mail from John," unless you were referring to a specific article of mail. While it wouldn't be *incorrect* to do so, you'd typically just say "I received (some) mail from John," I would think.

I suppose the kicker would be when you wish to refer to a specific e-mail under the rules of the noun mail. The thing is, the form "electronic mail" refers more to the system of transmission than individual transmissions themselves. While you could probably drum up a term to take the place of "piece," this very situation just might be what caused the creation of the form of e-mail, independent of its root, meaning "specific article of e-mail," bringing us back to where we began.

So, I think this is probably just a simple case of etymological evolution- something you see frequently in the ever-developing world of technology. Regardless, no one's going to trounce you for using "e-mails."