Checked it out, and it’s simpler than I thought.

Screen caps of email. The first is as it arrived in the Inbox, the second in the Junk folder.

Basically, they have an inline HTML file (and an inline spotify png). The HTML has lots of embedded rubbish text, associated with a specific style. That style is set to display:none, so it is hidden.

Now by default Thunderbird shows me the inline images and css. As it’s set none, I don’t see the rubbish text and the message looks and reads like a normal message.

But when it’s marked as Junk, Thunderbird won’t show the image, and won’t show any css. So the message then displays all the rubbish text and it looks garbled.

eg: the Bold heading in the body of the email is actually the html here