Before you hit that send button proofread what you have written. Re-read to make sure it says what you think it does, check your grammar, check your spelling (not just with spellcheck which is often wrong) and most of all make sure what you have written is clear, concise and easy to understand.
You want to convey the impression that you are an articulate, intelligent and professional business person. You also want the recipient of your email to know that you respect them and their time. Taking a little more of your own time to proofread and make sure your emails sound professional goes a long way toward both of these ends.