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!