Here's my favourite answer to that question. It's taken from The Virtual Tutor:
We are witnessing the introduction of a new word into the English language, and it may take some more time for its use to be agreed upon by everyone who uses it. In my experience, "electronic mail" has gone from E-mail to e-mail to email, and it is regularly used as a countable noun. The fact that "mail" is still a noncount noun does not seem to affect "email" at all. I think this is because with mail, we say "letter" to talk about an individual unit, but with email, we don't have a word for an individual unit. "Message" became confused with phone or voice mail messages, and no one used e-letter, so email has become the count noun. I predict that the shortest spelling (email) will soon be the standard. I hope this answers your question.
In short, it's treated as a countable noun and as a non-countable noun. You have a choice. How do the people around you use it?
All the best.