Avoid extremes of style. Messages that are too formal are just as bad as messages that are too informal.
Don't use formal expressions such as the following:
I am in receipt of your e-mail of Monday, September 3. I look forward to hearing from you at your earliest convenience. Thanking you in advance.
Likewise, don't imitate conversational style such as this:
Hi, there! How are you doing? Well, whaddya know? Gee, you didn't tell me you bought a new car!