In many cases, it's the first thing that will get a reader's attention or make the reader ignore your message. Be careful about spelling and punctuation. Try to follow the same rules you'd use if you were writing a letter or a memo. Some folks equate items in uppercase letters with shouting. Make your message as short as possible, but don't make it cryptic or unclear.
Most users have to deal with limits on the amount of email they can receive.
Keep the body of the message succinct. Limit a message to one or two screens. Write relatively short paragraphs and limit lines to 75 characters. Some email programs wrap long lines at whatever the window margin is set. Others don't, Give your reader a break and wrap the lines for them.