You know my feelings on this: email can sometimes cause a lot of problems in a friendship. This is especially true if you use email as a way to bring up an uncomfortable subject.
You know what happens, right? Your friend doesn't always get that you're kidding, or that you're serious, or they think you are accusing them of something... they don't get to see your face or hear the tone in your voice, and as a result that email makes everything worse than it needs to be.
Has a misconstrued email gotten you into trouble with a friend?