I run two email addresses: one business, one personal. As you may imagine, the emails I get between the two are quite different. I'm using Thunderbird and always, when given the choice, opt for plain text email. I find Thunderbird does a very good job of rendering HTML email legible (easy to read) and functional (links work, etc).

Perhaps once a month, maybe less, I have to use the toolbar button "Show HTML Temp" to do a one-off rendering of an email as basic HTML. Almost always the same offenders.

I find it such a success that I recommend it to my customers - none of which are computer experts, and most are not what I'd call "computer savvy". A process has to meet a high bar for me to recommend something like this to my customers.

Shame that when I explain how it can help in improving one's security that a vanishingly-small number of customers take me up on the offer!