That probably depends on whether you want to convey meaning by formatting of free flowing text. Professional way to do that is to highlight (highlight, not make

That probably depends on whether you want to convey meaning by formatting of free flowing text. Professional way to do that is to highlight (highlight, not make bold, italics or whatever) the parts of your email that constitute the tl;dr part, even more professional is to simply write your email such that it's main point is repeated in first and/or last paragraph.