Yes, I disallow HTML as well. Well, technically, I don't allow HTML to render. If the email is important, then I'll read the HTML source myself and extract the parts that are important.

Because mutt?

No, I actually use Thunderbird. I disallow HTML for security reasons. It lets me avoid having any remote resources (such as images, etc.) load and eliminates the possibility of any sneaky JS or somesuch executing.

Is this a default setting I'm somehow missing?

I can choose to send mails as Plain Text but disable rendering of HTML mails? Where do I set that up?

View > Message Body As > Plain Text

Ok, will check this later. Thanks!

Why??

Because HTML mails are almost always spam anyway.

And disabling HTML should help you avoid tracking pixels which tell the sender if you opened the email or not, etc.

You don't need to disable HTML for that. Thunderbird blocks remote content in HTML by default and only displays images embedded in the email. Isn't the tracking pixel mostly snake oil since it mostly tell if the user use gmail (preloading images) or an email client that block remote content by default?

See my reply to sam_lowry above. Short answer: security.