People try to do too much in their emails. If you want a bunch of fancy crap then include a hyperlink to a webpage and have all of the goodness up there. I implemented a policy about 15 years back of only allowing plain text emails and a plain text style guide to go along with it.
Clients love it. All killer no filler and displays the way its meant to even on a command line mail client. It's got to the point where I would probably get a bunch of push back if I switched to html email now.
Of course, we don't send any advertising type emails.