Make sure your subject lines make sense. Busy people have busy email boxes. Don't be cute or make them wonder what you are talking about. Be concise and informative. Email is not a formal means of communication-yet.
Email is still a relatively informal means of communication.
If you have something that is extremely important, use the old US Postal Service. It may be slow, but you can request a return receipt and certify its receipt to the proper person. Use spell check and grammar check. If your email program doesn't have either of these, compose your messages in a word processing program and copy and paste it into your email message. There is nothing worse than misspelled words or incomplete sentences.