As someone who works on an email system, it's a bit of a shame that this didn't win over HTML at least for email between humans. HTML is a bit of nightmare to sanitize. It's also expensive to layout compared to something like text/enriched which wouldn't need a full WebView on mobile - you'd be able to easily layout using native view widgets. Especially when you consider HTML in emails is stuck on a mostly old school table based layout system for highly styled emails, and a lot of these emails are not responsive.