Tim Bray illustrates why current email client support for HTML enriched mail just plain sucks.

This Is War Just now, a couple of spams that made it through Mozilla were in the inbox, and I missed slightly with the bold swipe of the mouse to group-select for erasure and actually selected one, which brought it up in the message pane, a pitch for strapless bras. Suddenly my computer was talking to me: "For years, women have struggled..." then my finger reached the Del key and it was gone. Lauren raised an eyebrow at me from across the room and when I explained said “I think we’ve just hit a new low.” She’s so diplomatic.

I've always hated HTML enriched mail. Why do client vendors think that the very same people that design ugly-ass websites using Front Page should be allowed to inflict that type of pain and suffering on their friends and colleagues? Why do they assume that the same low-lifes that have no problem flooding you with pop-up windows will somehow behave themselves when they spam you?
My solution?
Thunderbird.
Thunderbird allows you to modify a lot of its default behaviors by getting under the hood. This isn't terribly difficult stuff because all that "getting under the hood" consists of is editing a text file.
By adding this...

// HTML messages display (0 - HTML, 1 - plain text):
user_pref("mailnews.display.html_as", 1);

...to my users.js file, I force Thunderbird to display all email that I read as "pure" text - no more HTML. There are a couple of wonderful implications to this. First, there's no way that I'll be forced to listen to bra pitchmen when I read that next message, and two, I'm not constantly download images off of remote web servers and verifying that my email address is active.
To quote the immortal words of one Mr. Scooby J. Doo, "I rike it, I rike it a rot!"