Busy users do not have time to read all mail at one sitting. The subject line allows users to scan for messages with the highest priority. If you are responding to a previous message, summarize that message before you reply. This helps your reader remember their original comments. Use upper and lower case letters in your messages.
All upper case is harder to read.
Do not abuse the power of electronic mail. Do not use mailing lists as your personal advertising billboard. Mailing lists are for exchange of information, not selling or marketing unless it specifically says this is okay. Use special symbols or words to indicate humor or an emotional response. Such indications alert the reader that you realize you are being emotional.