How can I stop Spotlight searching my email?

It's funny how infrequently I think of using Spotlight, actually, because every time I experiment with it, I'm delighted by the results and think "cool, I should use this more often." Seems like you've had that same thought and made it stick. Good job! :-)

In terms of your question, I know exactly what you mean. Here's a sample search for my friend and colleague Ken McCarthy. Notice the categories of matched entries:

You can see an entire section of email messages have been matched.

To turn this off is pretty simple, fortunately. Choose the "Spotlight preferences..." at the bottom of the menu and you'll be whisked away to the Spotlight System Preference:

To stop email messages being included in your search results, simply uncheck "Mail Messages" and close the window up. I actually prefer to filter out matches that are Bookmarks, Music, Movies, Fonts and Presentations too, just to avoid false hits and confusing results.

Now when I do the very same search, the results omit email messages as I'd like:

That should get your Spotlight configured as you seek!