If you sent out a newsletter in full-fledged, busy HTML with embedded images and bells and whistles, I wouldn't be able to read it in mutt, which means I probably wouldn't read it at all. For me it's just a convenience factor, but Arnie's right to mention security as an issue, and it's a concern for accessibility as well.

If you go with the majority (so far) opinion and opt for full HTML email, I hope that at the very least you will include *at the top* a text blurb to the effect of, "If you can't read HTML email, you can find this issue of the newsletter on the web at <URL>." But I would strongly urge you to compromise and have a text email with lots of links to further info and pretty pictures on the website.