Mu4e can render HTML e-mails in Emacs (using Emacs’ shr render engine). Works well enough for most cases. When it doesn’t, there is a command for opening the message in your browser of choice. Re sending HTML: I personally think it’s a bad idea and don’t do it. But I think it‘s possible to write e-mails in org mode markup and them have them automatically converted to HTML. Probably easy to build something similar for Markdown, if it doesn’t exist yet.