I'm not sure what you're doing, so my answer is based on a guess.
When you're typing an email and you get to the far right side of the text box, are you hitting the enter key if the last word on each line doesn't fit? If you are, you don't want to do that. Just keep typing, and if a word doesn't fit on the end of the line it will automatically wrap around to the next line. Hitting enter once will create a new paragraph.