Or, those bloody pine/mutt users.
In my experience there are two largish pools of residual ASCII-only email users:
- People who first got on email back when ASCII-only was the only option and have never bothered to update.
- People who first got on email back when ASCII-only was the only option, and vehemently hate HTML in email (it's dangerous!) and actively resist changing their reading mode.
This is one of the areas where sysadmins tend to show off their inherent conservatism, though I've noticed that this twitch only extends to sysadmins who were around back in the ASCII-only Internet (very roughly, anyone born after 1980 is not likely to be such a person). A lot of us considered our preferences vindicated in the wake of such memorable events as Melissa and Anna Kournikova that completely broke whole email systems, as text-mode users were effectively immune to them(1). We may have a reputation as gadget fiends, but a sizable percentage of us only use GUIs when absolutely unavoidable (Windows Core was made for these people).
With the advent of ubiquitous webmail and the cost of doing email right increasing every year forcing ever larger companies to outsource their email handling to Google or Microsoft, the HTML-in-email boat has sailed. Screen readers, and outright mail readers, for the blind used to be an argument against HTML in mail but even those can handle it in these advanced-computing times. Sure, things like bullets are hard to read, but then, text-mode bullets are just as hard:
The problems from this weekend were: * Too many cooks in the kitchen. * One too many recipe books. More than one in fact. * An out-dated copy of the recipe in someone's hands. * This resulted in too little salt. * Frank refuses to ever eat anything we make again. * Who forgot the plates??
Where the problems come in is using advanced formatting, such as you find in all of those 'newsletters' you get whenever you sign up for a site for any reason. Those are barely intelligible to such readers. Mail with bolding, italics, underlines, lists and indents is quite machine-readable now.
This brings up another sub-set, the text-rendering preference; the mail-reader is quite able to handle HTML but is set to display the plain-text part if one exists. This is most commonly experienced in organizations that allow email "stationary". In my experience, these are not HTML-deniers by in large, they just hate crimes against good formatting-sense(2).
As with anything, the choices you make sometimes get wrapped up in the perception of your own identity. I saw this in my Novell days, I'm seeing some of it now in some Windows users, and I've been seeing it in myself when it comes to Linux usage. By now, continued use of text-mode mail-readers are the result of laziness or identity. As a result, they're not going to change without a lot of convincing.
(1): In my case, I was working for a company using Novell GroupWise which broke the propagation vector. Still HTML-in-mail, but also dodged those bullets. The smug in Novell circles was mighty those weeks.
(2): Which I understand. I was one for ages.