Been using Mozilla’s Thunderbird 1.0 lately, and noticed a little weirdness in the way they hide images to protect your privacy. Just like most other email applications out there, it won’t automatically display images in an HTML email unless you click the "Show Images" button. However, most other applications at least make it look like something’s missing, like with big red "broken image" icons or something. Not so with Mozilla Thunderbird.
You wouldn’t think this is such a big deal, but it actually took me weeks before I realized all the HTML email I was getting was actually missing their logos! So, keep that in mind when you code your HTML email—perhaps use ALT-text that makes it obvious your images are missing ("MailChimp Logo").