So that is all I am asking, that we have the option to 'add' the <pre> tag to all messages we view online
I'd recommend against <pre>, as it has the primary effect of eliminating word-wrap and replacing it with a horizontal scroll (when the source line is too long for the display window).
Rather I'd suggest what you started with: use a fixed pitch font.
I think it makes sense to apply to messages that have only a text/plain message body. I'm not sure if it would be useful or just annoying to apply it as a default font for text/HTML message bodies.
