Some html code to do this is described here: http://codeverge.com/mozilla.support.thunderbird/line-spacing-how-to-double-space/1402587 Personally I'd put it into a template, or use Stationery, or set up a userContent.css file in the profile. It works here for me in Thunderbird, but in truth what is seen is determined by the software used by the recipient. If they choose to use plain text mode it won't have any effect. I thought, and maybe I am wrong, that the point of double-spaced copy was to allow easy notation and amendments. That's fine if you're doing this by hand, on paper hardcopy, but the double spacing we have here, applied via html, would preclude any typed editing into the gaps between the lines of text.