When my spouse opens a message she regularly, but occasionally, gets an email that shows the body completely in html (all tags are shown) and the subject is "DKIM internal error." I'll enclose a screen shot of the first part of an email to illustrate. This makes the message very difficult to read. Oddly, you'll see that the first link is a reference to americatestkitchen.com, but the message was sent by someone completely unrelated to americatestkitchen. Perhaps it was an add inserted by the sender's email server. The headers make it appear that the message came from netflix, but the sender is completely unrelated to netflix. There is no actual content in the html. Theh html appears to be defective (there are no ending tags at the end of the message). Closing and reopening Thunderbird doesn't fix it. Most messages display fine, but some do not.