Not the original poster, but although a lot of Emacs users don't use HTML email, it's certainly possible to do it. I wrote an Emacs package that allowed me to

Not the original poster, but although a lot of Emacs users don't use HTML email, it's certainly possible to do it. I wrote an Emacs package that allowed me to render my emails in that way Outlook does it (comments on top, everything HTML). I write my emails using markup, and then it's all rendered prior to sending. I had people mention how pretty my emails were. HTML emails also render fine, with all images as you would expect. Emacs does support online images.