You say "forward" but are you sure you don't mean "reply"? And do you mean vertical coloured lines in the margin, or horizontal rules? The sort of indentation you describe is typical of ''replying'', preserving the sense of who said what and when. In my experience, Thunderbird ''forwards'' by simply pasting in the original message, complete with a summary of the to/from/subject header parts. I can suggest two addons which adjust what happens when you reply. https://freeshell.de/~kaosmos/changequote-en.html https://freeshell.de/~kaosmos/quoteandcomposemanager-en.html