Nearly all marketing messages that I receive are HTML only, not multipart. In fact, this is a problem that seems almost as if it is specific to marketing spam; ordinary people who send HTML mail seem to use MUAs that sensibly send a plain text part as well. But whatever tools marketers use to send their "newsletters" seem to be ignorant of best practices, or they just don't care about the small market segment of people who don't read HTML mail.

This is not a complaint. I don't care to read what they're sending me anyway, and the wall of visual garbage that greets me when I open one of those messages has trained me to reflexively hit delete and move on.