I asked David Blatner, who runs the InDesign Secrets website, the same question about creating HTML Newsletters with Muse in a future edition, and he replied:
"The problem with "html email" is that it is dependent on very old email clients. For example, we are having a problem right now that some (but not all) people using Outlook 2003 are having trouble reading the free InDesign Tip of the Week email that we send using mailchimp. Yes, Outlook 2003.
Basically, html email has to be super lowest common denominator HTML. The best html email wouldn't even use CSS, but most email clients can read very simple css now. But remember that later versions of Outlook use MS Word as their HTML viewer, so… it's painful. That means, you really want to use tables and shim.gifs and stuff like that. It's not pretty.
Muse and in5 and most other tools are interested in creating 21st century HTML, not html from 1998. That's why it's extremely unlikely we'll see anything like this from Adobe (or probably even anyone else)."