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. Include only the portions pertinent to your reply. You do this to set a context for your message. Include a signature with all your email. This ought to include your full name and some information about how to contact you by telephone or your mail address.