Email should be text anyway. I =hate= html formatted email, as rendering is always an iffy thing.
It sure solves a lot of problems that surround email, if all you get is raw ascii text. phishing is much more difficult if you can't obfuscate the actual link address.
Exactly why HTML mail is being filtered, not yet everywhere, but probably eventually. The Chinese are phishing using messages that look like official DoD correspondence, which are of course, quite successful.