Not started when? You hit enter perhaps, although in HTML terms of HTML that is a new paragraph, not a new line as HTML automatically wraps into the space it is provided (this can be quite a long line if you are using a wide aspect 120" television as your monitor.) But the same can be said of word processors, they insert a new paragraph when you hit the enter key and wrap paragraph text to fit the paper the printer is designated as having.

Thunderbird is on a 14inch laptop. I can only compose on 1 line using Body text or Paragraph Format. You can start a new line here without doing anything.