"If you find yourself in a hole, stop digging." -Will Rogers
Never have an attitude when writing an email. When we have an attitude in writing an email two things happen. The likelihood that the person on the other end will want to help us diminishes. And the likelihood that we will regret it later will increase. This works in person too.
Always assume that those you don't want to see your email...will see it. Invariably they will. And if by chance they don't see your email, you will still waste a lot of time later wondering if they did.
Important emails should marinate, especially ones you are tempted to begin with the salutation, "Listen Buster!" You will likely think of a better way to share your views.
This is perhaps the simplest test of all. Pretend that what you have to say is going to be the lead story in The Los Angeles Times, or your local paper, in the morning. And ask yourself this question. "If all my friends and clients were to see this will I be happy or sad?"